diff --git a/src/simplebuild_dgcode/data/pkgs/G4/G4HeatMap/pymod__impl/mod.cc b/src/simplebuild_dgcode/data/pkgs/G4/G4HeatMap/pymod__impl/mod.cc
index 40fd126..1ff559e 100644
--- a/src/simplebuild_dgcode/data/pkgs/G4/G4HeatMap/pymod__impl/mod.cc
+++ b/src/simplebuild_dgcode/data/pkgs/G4/G4HeatMap/pymod__impl/mod.cc
@@ -288,7 +288,7 @@ namespace DMWriter {
     try {
       m_eval_quantity = m_expr_builder.createEvaluator<ExprParser::float_type>(m_expr_quantity);
     } catch (ExprParser::InputError& e) {
-      printf("\nHeatMapWriter ERROR: Invalid quantity expression \"%s\"\n",m_expr_filter.c_str());
+      printf("\nHeatMapWriter ERROR: Invalid quantity expression \"%s\"\n",m_expr_quantity.c_str());
       printf("HeatMapWriter ERROR: %s : %s\n\n",e.epType(),e.epWhat());
       throw;
     }
diff --git a/src/simplebuild_dgcode/data/pkgs/MCPL/G4MCPLPlugins/pymod__MCPLWriter/mod.cc b/src/simplebuild_dgcode/data/pkgs/MCPL/G4MCPLPlugins/pymod__MCPLWriter/mod.cc
index af7dc4f..14f3c58 100644
--- a/src/simplebuild_dgcode/data/pkgs/MCPL/G4MCPLPlugins/pymod__MCPLWriter/mod.cc
+++ b/src/simplebuild_dgcode/data/pkgs/MCPL/G4MCPLPlugins/pymod__MCPLWriter/mod.cc
@@ -533,7 +533,7 @@ namespace G4MCPLWriter {
         if (!m_expr_flags.empty())
           m_eval_flags = m_expr_builder->createEvaluator<EP::int_type>(m_expr_flags);
       } catch (EP::InputError& e) {
-        printf("\nMCPLWriter ERROR: Invalid expression for user-flas \"%s\"\n",m_expr_flags.c_str());
+        printf("\nMCPLWriter ERROR: Invalid expression for user flags \"%s\"\n",m_expr_flags.c_str());
         printf("MCPLWriter ERROR: %s : %s\n\n",e.epType(),e.epWhat());
         if (m_sd)
           m_sd->abort();
diff --git a/src/simplebuild_dgcode/data/pkgs_val/UnitTests/G4MCPLTests/pkg.info b/src/simplebuild_dgcode/data/pkgs_val/UnitTests/G4MCPLTests/pkg.info
index 9971f72..ae03ce2 100644
--- a/src/simplebuild_dgcode/data/pkgs_val/UnitTests/G4MCPLTests/pkg.info
+++ b/src/simplebuild_dgcode/data/pkgs_val/UnitTests/G4MCPLTests/pkg.info
@@ -1,4 +1,4 @@
-package(USEPKG G4Launcher G4MCPLPlugins MCPLExtra MCPLTestData G4StdGeometries G4StdGenerators USEEXT MCPL)
+package(USEPKG G4Launcher G4MCPLPlugins G4HeatMap Mesh3D MCPLExtra MCPLTestData G4StdGeometries G4StdGenerators USEEXT MCPL)
 
 ##########################################################
 
diff --git a/src/simplebuild_dgcode/data/pkgs_val/UnitTests/G4MCPLTests/scripts/testexpressions b/src/simplebuild_dgcode/data/pkgs_val/UnitTests/G4MCPLTests/scripts/testexpressions
new file mode 100755
index 0000000..91fdbf8
--- /dev/null
+++ b/src/simplebuild_dgcode/data/pkgs_val/UnitTests/G4MCPLTests/scripts/testexpressions
@@ -0,0 +1,102 @@
+#!/usr/bin/env python3
+
+#Tests of expressions (G4ExprParser, MCPLExprParser) given to MCPLGen,
+#MCPLWriter (--mcpl), G4HeatMap (--heatmap) and the sb_mcplextra_filterfile
+#and sb_mcplextra_filterview commands.
+#
+#The expressions are parsed and evaluated in the ExprParser libraries, but used
+#(and errors are caught) in Python modules. This tests that this works across
+#modules, also on platforms which compare type information by address (e.g.
+#Intel macOS):
+#
+#  * Valid expressions of various value types (bool, integer, floating point,
+#    string) work.
+#  * Invalid expressions (syntax errors, unknown variables, domain errors) give
+#    a clean error message (the ExprParser exception is caught by its base class
+#    in another module), rather than a crash.
+
+import subprocess
+import pathlib
+import mcpl
+
+infile = 'MCPLTestData/miscphys.mcpl.gz'
+infile_path = subprocess.run( [ 'sb_core_finddata', 'MCPLTestData', 'miscphys.mcpl.gz' ],
+                              capture_output = True, text = True, check = True ).stdout.strip()
+
+def run( title, cmd, expect_ok, markers ):
+    print()
+    print( f'==> {title}' )
+    r = subprocess.run( cmd, capture_output = True, text = True, check = False )
+    out = r.stdout + r.stderr
+    #Only show the relevant messages:
+    for line in out.splitlines():
+        for m in markers:
+            if m in line:
+                print( '   ', line[line.index(m):].strip() )
+                break
+    #A negative return code means the process was killed by a signal (e.g. an
+    #uncaught exception leading to abort()):
+    assert r.returncode >= 0, f'command crashed (signal {-r.returncode}):\n' + out
+    assert 'terminate called' not in out and 'libc++abi' not in out, 'uncaught exception:\n' + out
+    if expect_ok:
+        assert r.returncode == 0, 'command failed:\n' + out
+    else:
+        assert r.returncode != 0, 'command unexpectedly succeeded:\n' + out
+        print( '    (failed as expected)' )
+    return out
+
+def nparticles( fn ):
+    return mcpl.MCPLFile( fn ).nparticles
+
+def sim( title, *args, expect_ok = True ):
+    for f in pathlib.Path('.').glob('out.mcpl*'):
+        f.unlink()
+    run( title, [ 'sb_g4mcpltests_mcplstatsums', f'input_file={infile}' ] + list(args),
+         expect_ok, ( 'MCPLGen ERROR', 'MCPLWriter ERROR', 'HeatMapWriter ERROR' ) )
+    if expect_ok and pathlib.Path('out.mcpl.gz').exists():
+        print( f'    Output particles: {nparticles("out.mcpl.gz")}' )
+
+#Valid expressions of different value types, in MCPLGen, MCPLWriter and
+#G4HeatMap:
+sim( 'MCPLGen filter (floating point)', 'input_filter=ekin > 1MeV',
+     '--mcpl=grabsrc to out.mcpl' )
+sim( 'MCPLGen filter (integer)', 'input_filter=pdgcode == 2112',
+     '--mcpl=grabsrc to out.mcpl' )
+sim( 'MCPLGen filter (bool, combined)', 'input_filter=is_neutron && neutron_wl > 1Aa',
+     '--mcpl=grabsrc to out.mcpl' )
+sim( 'MCPLWriter filter (integer)', '--mcpl=grabsrc where trk.pdgcode == 22 to out.mcpl' )
+sim( 'MCPLWriter filter (floating point)', '--mcpl=grabsrc where step.pre.ekin > 1 MeV to out.mcpl' )
+sim( 'MCPLWriter filter (string)', '--mcpl=grabsrc where step.volname == "World" to out.mcpl' )
+sim( 'MCPLWriter user flags (integer)', '--mcpl=grabsrc withflags trk.pdgcode to out.mcpl' )
+sim( 'Heat map (floating point, with filter)',
+     '--heatmap=step.steplength where trk.is_neutron to heatmap' )
+import Mesh3D
+print( '    Heat map events: %i'%Mesh3D.Mesh3D( 'heatmap.mesh3d' ).stats['nevts'] )
+
+#Invalid expressions:
+sim( 'MCPLGen filter with syntax error', 'input_filter=ekin >', expect_ok = False )
+sim( 'MCPLGen filter with unknown variable', 'input_filter=nosuchvariable > 1',
+     expect_ok = False )
+sim( 'MCPLGen filter with domain error', 'input_filter=ekin > 1/0', expect_ok = False )
+sim( 'MCPLWriter filter with syntax error', '--mcpl=grabsrc where trk.pdgcode == to out.mcpl',
+     expect_ok = False )
+sim( 'MCPLWriter filter with unknown variable', '--mcpl=grabsrc where trk.nosuchvariable to out.mcpl',
+     expect_ok = False )
+sim( 'MCPLWriter user flags with unknown variable',
+     '--mcpl=grabsrc withflags trk.nosuchvariable to out.mcpl', expect_ok = False )
+sim( 'Heat map with unknown variable', '--heatmap=step.nosuchvariable to heatmap',
+     expect_ok = False )
+sim( 'Heat map filter with syntax error', '--heatmap=step.steplength where trk.pdgcode == to heatmap',
+     expect_ok = False )
+
+#The sb_mcplextra_filterfile and sb_mcplextra_filterview commands:
+def mcplextra( title, cmd, expect_ok ):
+    run( title, cmd, expect_ok, ( 'in filter expression', 'Error', 'ERROR' ) )
+mcplextra( 'sb_mcplextra_filterfile', [ 'sb_mcplextra_filterfile', '-n',
+                                        infile_path, 'filtered.mcpl', 'is_neutron' ], True )
+print( f'    Output particles: {nparticles("filtered.mcpl")}' )
+mcplextra( 'sb_mcplextra_filterfile with invalid expression',
+           [ 'sb_mcplextra_filterfile', '-n', infile_path, 'filtered2.mcpl', 'ekin >' ], False )
+mcplextra( 'sb_mcplextra_filterview', [ 'sb_mcplextra_filterview', 'is_neutron', infile_path ], True )
+mcplextra( 'sb_mcplextra_filterview with invalid expression',
+           [ 'sb_mcplextra_filterview', 'nosuchvariable > 1', infile_path ], False )
diff --git a/src/simplebuild_dgcode/data/pkgs_val/UnitTests/G4MCPLTests/scripts/testexpressions.log b/src/simplebuild_dgcode/data/pkgs_val/UnitTests/G4MCPLTests/scripts/testexpressions.log
new file mode 100644
index 0000000..6d90edb
--- /dev/null
+++ b/src/simplebuild_dgcode/data/pkgs_val/UnitTests/G4MCPLTests/scripts/testexpressions.log
@@ -0,0 +1,74 @@
+
+==> MCPLGen filter (floating point)
+    Output particles: 110
+
+==> MCPLGen filter (integer)
+    Output particles: 15
+
+==> MCPLGen filter (bool, combined)
+    Output particles: 10
+
+==> MCPLWriter filter (integer)
+    Output particles: 15
+
+==> MCPLWriter filter (floating point)
+    Output particles: 110
+
+==> MCPLWriter filter (string)
+    Output particles: 195
+
+==> MCPLWriter user flags (integer)
+    Output particles: 195
+
+==> Heat map (floating point, with filter)
+    Heat map events: 195
+
+==> MCPLGen filter with syntax error
+    MCPLGen ERROR - ParseError in filter expression : missing arguments for operator : >
+    (failed as expected)
+
+==> MCPLGen filter with unknown variable
+    MCPLGen ERROR - ParseError in filter expression : Unknown or badly placed identifier : nosuchvariable
+    (failed as expected)
+
+==> MCPLGen filter with domain error
+    MCPLGen ERROR - DomainError in filter expression : Division with value which is always zero
+    (failed as expected)
+
+==> MCPLWriter filter with syntax error
+    MCPLWriter ERROR: Invalid filter expression "trk.is_primary && ( step.stepnbr==1 && (trk.pdgcode ==) )"
+    MCPLWriter ERROR: ParseError : missing arguments for operator : &&
+    (failed as expected)
+
+==> MCPLWriter filter with unknown variable
+    MCPLWriter ERROR: Invalid filter expression "trk.is_primary && ( step.stepnbr==1 && (trk.nosuchvariable) )"
+    MCPLWriter ERROR: ParseError : unknown trk property : "nosuchvariable"
+    (failed as expected)
+
+==> MCPLWriter user flags with unknown variable
+    MCPLWriter ERROR: Invalid expression for user flags "trk.nosuchvariable"
+    MCPLWriter ERROR: ParseError : unknown trk property : "nosuchvariable"
+    (failed as expected)
+
+==> Heat map with unknown variable
+    HeatMapWriter ERROR: Invalid quantity expression "step.nosuchvariable"
+    HeatMapWriter ERROR: ParseError : unknown step property : "nosuchvariable"
+    (failed as expected)
+
+==> Heat map filter with syntax error
+    HeatMapWriter ERROR: Invalid filter expression "trk.pdgcode =="
+    HeatMapWriter ERROR: ParseError : missing arguments for operator : ==
+    (failed as expected)
+
+==> sb_mcplextra_filterfile
+    Output particles: 15
+
+==> sb_mcplextra_filterfile with invalid expression
+    in filter expression : missing arguments for operator : >
+    (failed as expected)
+
+==> sb_mcplextra_filterview
+
+==> sb_mcplextra_filterview with invalid expression
+    in filter expression : Unknown or badly placed identifier : nosuchvariable
+    (failed as expected)