diff --git a/src/main/java/dk/camelot64/kickc/passes/Pass1GenerateSingleStaticAssignmentForm.java b/src/main/java/dk/camelot64/kickc/passes/Pass1GenerateSingleStaticAssignmentForm.java
index f4b7bf362..193fa6f13 100644
--- a/src/main/java/dk/camelot64/kickc/passes/Pass1GenerateSingleStaticAssignmentForm.java
+++ b/src/main/java/dk/camelot64/kickc/passes/Pass1GenerateSingleStaticAssignmentForm.java
@@ -45,9 +45,16 @@ public class Pass1GenerateSingleStaticAssignmentForm extends Pass1Base {
                   if (assignedVar instanceof VariableUnversioned) {
                      // Assignment to a non-versioned non-intermediary variable
                      VariableUnversioned assignedSymbol = (VariableUnversioned) assignedVar;
-                     VariableVersion version = assignedSymbol.createVersion();
+                     VariableVersion version;
                      if(assignedSymbol.isDeclaredConstant()) {
+                        Collection<VariableVersion> versions = assignedVar.getScope().getVersions(assignedSymbol);
+                        if(versions.size()!=0) {
+                           throw new CompileError("Error! Constants can not be modified "+statement);
+                        }
+                        version = assignedSymbol.createVersion();
                         version.setDeclaredConstant(true);
+                     } else {
+                        version = assignedSymbol.createVersion();
                      }
                      statementLValue.setlValue(version.getRef());
                   }
@@ -173,7 +180,7 @@ public class Pass1GenerateSingleStaticAssignmentForm extends Pass1Base {
                Scope scope = rSymbol.getScope();
                Collection<VariableVersion> versions = scope.getVersions(rSymbol);
                if(versions.size()!=1) {
-                  throw new RuntimeException("Error! Constants always must exactly one version "+rSymbol);
+                  throw new CompileError("Error! Constants must have exactly one version "+rSymbol);
                }
                return versions.iterator().next();
             }  else {
diff --git a/src/main/java/dk/camelot64/kickc/test/TestPrograms.java b/src/main/java/dk/camelot64/kickc/test/TestPrograms.java
index d7f14de9e..3d6f0c65e 100644
--- a/src/main/java/dk/camelot64/kickc/test/TestPrograms.java
+++ b/src/main/java/dk/camelot64/kickc/test/TestPrograms.java
@@ -28,7 +28,6 @@ public class TestPrograms extends TestCase {
       compileAndCompare("constabsmin");
    }
 
-
    public void testBasicFloats() throws IOException, URISyntaxException {
       compileAndCompare("basic-floats");
    }
@@ -258,6 +257,16 @@ public class TestPrograms extends TestCase {
       compileAndCompare("forrangemin");
    }
 
+   public void testAssignConst() throws IOException, URISyntaxException {
+      try {
+         compileAndCompare("assign-const");
+      } catch (CompileError e) {
+         // expecting error!
+         return;
+      }
+      fail("Expected compile error.");
+   }
+
    public void testStmtOutsideMethod() throws IOException, URISyntaxException {
       try {
          compileAndCompare("stmt-outside-method");
diff --git a/src/main/java/dk/camelot64/kickc/test/assign-const.kc b/src/main/java/dk/camelot64/kickc/test/assign-const.kc
new file mode 100644
index 000000000..7bd3167f5
--- /dev/null
+++ b/src/main/java/dk/camelot64/kickc/test/assign-const.kc
@@ -0,0 +1,7 @@
+const byte prime = 7;
+
+void main() {
+    prime = 13;
+    byte* screen = $0400;
+    *screen = prime;
+}
\ No newline at end of file