diff --git a/src/thingdef/thingdef_expression.cpp b/src/thingdef/thingdef_expression.cpp
index ce603f261..695b12799 100644
--- a/src/thingdef/thingdef_expression.cpp
+++ b/src/thingdef/thingdef_expression.cpp
@@ -3373,6 +3373,19 @@ FxExpression *FxClassMember::Resolve(FCompileContext &ctx)
 
 ExpEmit FxClassMember::Emit(VMFunctionBuilder *build)
 {
+	if (~membervar->Flags & VARF_Native)
+	{	// Check if this is a user-defined variable.
+		// As of right now, FxClassMember is only ever used with FxSelf.
+		// This very user variable was defined in stateowner so if
+		// self (a0) != stateowner (a1) then the offset is most likely
+		// going to end up being totally wrong even if the variable was
+		// redefined in self which means we have to abort to avoid reading
+		// or writing to a random address and possibly crash.
+		build->Emit(OP_EQA_R, 1, 0, 1);
+		build->Emit(OP_JMP, 1);
+		build->Emit(OP_THROW, 2, X_BAD_SELF);
+	}
+
 	ExpEmit obj = classx->Emit(build);
 	assert(obj.RegType == REGT_POINTER);
 
diff --git a/src/zscript/vm.h b/src/zscript/vm.h
index 0d45ad90a..a000e9533 100644
--- a/src/zscript/vm.h
+++ b/src/zscript/vm.h
@@ -163,6 +163,7 @@ enum EVMAbortException
 	X_TOO_MANY_TRIES,
 	X_ARRAY_OUT_OF_BOUNDS,
 	X_DIVISION_BY_ZERO,
+	X_BAD_SELF,
 };
 
 class VMFunction : public DObject
diff --git a/src/zscript/vmdisasm.cpp b/src/zscript/vmdisasm.cpp
index 35eb6085d..4e6f33ec7 100644
--- a/src/zscript/vmdisasm.cpp
+++ b/src/zscript/vmdisasm.cpp
@@ -371,7 +371,18 @@ void VMDisasm(FILE *out, const VMOP *code, int codesize, const VMScriptFunction
 			col = print_reg(out, 0, a, (mode & MODE_ATYPE) >> MODE_ASHIFT, 24, func);
 			if ((mode & MODE_BCTYPE) == MODE_BCTHROW)
 			{
-				mode = (code[i].a == 0) ? (MODE_BP | MODE_CUNUSED) : (MODE_BKP | MODE_CUNUSED);
+				if (code[i].a == 0)
+				{
+					mode = (MODE_BP | MODE_CUNUSED);
+				}
+				else if (code[i].a == 1)
+				{
+					mode = (MODE_BKP | MODE_CUNUSED);
+				}
+				else
+				{
+					mode = (MODE_BCJOINT | MODE_BCIMMS);
+				}
 			}
 			else if ((mode & MODE_BCTYPE) == MODE_BCCATCH)
 			{
diff --git a/src/zscript/vmexec.h b/src/zscript/vmexec.h
index 42a374030..adf2986c5 100644
--- a/src/zscript/vmexec.h
+++ b/src/zscript/vmexec.h
@@ -616,12 +616,16 @@ begin:
 			ASSERTA(B);
 			throw((VMException *)reg.a[B]);
 		}
-		else
+		else if (a == 1)
 		{
 			ASSERTKA(B);
 			assert(konstatag[B] == ATAG_OBJECT);
 			throw((VMException *)konsta[B].o);
 		}
+		else
+		{
+			THROW(BC);
+		}
 		NEXTOP;
 	OP(CATCH):
 		// This instruction is handled by our own catch handler and should
diff --git a/src/zscript/vmframe.cpp b/src/zscript/vmframe.cpp
index 29da0c5b9..50c7d78d9 100644
--- a/src/zscript/vmframe.cpp
+++ b/src/zscript/vmframe.cpp
@@ -440,6 +440,10 @@ int VMFrameStack::Call(VMFunction *func, VMValue *params, int numparams, VMRetur
 		case X_DIVISION_BY_ZERO:
 			Printf("division by zero.");
 			break;
+
+		case X_BAD_SELF:
+			Printf("invalid self pointer.");
+			break;
 		}
 		Printf("\n");
 
diff --git a/src/zscript/vmops.h b/src/zscript/vmops.h
index decb0b93d..39d224f5c 100644
--- a/src/zscript/vmops.h
+++ b/src/zscript/vmops.h
@@ -83,7 +83,8 @@ xx(RETI,	reti,	I8I16),		// Copy immediate from BC to return value A, possibly re
 xx(TRY,		try,	I24),		// When an exception is thrown, start searching for a handler at pc + ABC
 xx(UNTRY,	untry,	I8),		// Pop A entries off the exception stack
 xx(THROW,	throw,	THROW),		// A == 0: Throw exception object pB
-								// A != 0: Throw exception object pkB
+								// A == 1: Throw exception object pkB
+								// A >= 2: Throw VM exception of type BC
 xx(CATCH,	catch,	CATCH),		// A == 0: continue search on next try
 								// A == 1: continue execution at instruction immediately following CATCH (catches any exception)
 								// A == 2: (pB == <type of exception thrown>) then pc++ ; next instruction must JMP to another CATCH