diff --git a/src/main/java/dk/camelot64/kickc/Compiler.java b/src/main/java/dk/camelot64/kickc/Compiler.java
index 9c6d59f3a..a0a4cee11 100644
--- a/src/main/java/dk/camelot64/kickc/Compiler.java
+++ b/src/main/java/dk/camelot64/kickc/Compiler.java
@@ -170,6 +170,7 @@ public class Compiler {
       assertions.add(new Pass2AssertNoProcs(program));
       assertions.add(new Pass2AssertNoLabels(program));
       assertions.add(new Pass2AssertSingleAssignment(program));
+      assertions.add(new Pass2AssertRValues(program));
       for(Pass2SsaAssertion assertion : assertions) {
          assertion.check();
       }
diff --git a/src/main/java/dk/camelot64/kickc/passes/Pass2AssertRValues.java b/src/main/java/dk/camelot64/kickc/passes/Pass2AssertRValues.java
new file mode 100644
index 000000000..c234d72b7
--- /dev/null
+++ b/src/main/java/dk/camelot64/kickc/passes/Pass2AssertRValues.java
@@ -0,0 +1,38 @@
+package dk.camelot64.kickc.passes;
+
+import dk.camelot64.kickc.model.CompileError;
+import dk.camelot64.kickc.model.ControlFlowBlock;
+import dk.camelot64.kickc.model.ControlFlowGraphBaseVisitor;
+import dk.camelot64.kickc.model.Program;
+import dk.camelot64.kickc.model.statements.Statement;
+import dk.camelot64.kickc.model.statements.StatementCall;
+import dk.camelot64.kickc.model.values.ForwardVariableRef;
+import dk.camelot64.kickc.model.values.RValue;
+import dk.camelot64.kickc.model.values.VariableRef;
+
+import java.util.ListIterator;
+
+/** Asserts that the program does not contain any RValues that are forward-reference or unversioned */
+public class Pass2AssertRValues extends Pass2SsaAssertion {
+
+   public Pass2AssertRValues(Program program) {
+      super(program);
+   }
+
+   @Override
+   public void check() throws AssertionFailed {
+      ValueReplacer.executeAll(getGraph(), (replaceable, currentStmt, stmtIt, currentBlock) -> {
+         RValue rValue = replaceable.get();
+         if(rValue instanceof ForwardVariableRef) {
+            throw new CompileError("No forward references allowed "+currentStmt.toString(getProgram(), false));
+         }
+         if(rValue instanceof VariableRef) {
+            VariableRef variableRef = (VariableRef) rValue;
+            if(!variableRef.isIntermediate() && !variableRef.isVersion()) {
+               throw new CompileError("No unversioned variable references allowed "+currentStmt.toString(getProgram(), false));
+            }
+         }
+      });
+   }
+
+}