diff --git a/src/main/java/dk/camelot64/kickc/Compiler.java b/src/main/java/dk/camelot64/kickc/Compiler.java
index c5a6830c7..d28a2c9a4 100644
--- a/src/main/java/dk/camelot64/kickc/Compiler.java
+++ b/src/main/java/dk/camelot64/kickc/Compiler.java
@@ -118,6 +118,7 @@ public class Compiler {
       getLog().append("INITIAL CONTROL FLOW GRAPH");
       getLog().append(program.getGraph().toString(program));
 
+      new Pass1AssertUsedVars(program).execute();
       new Pass1ExtractInlineStrings(program).execute();
       new Pass1EliminateUncalledProcedures(program).execute();
       new Pass1EliminateUnusedVars(program).execute();
diff --git a/src/main/java/dk/camelot64/kickc/passes/Pass1AssertUsedVars.java b/src/main/java/dk/camelot64/kickc/passes/Pass1AssertUsedVars.java
new file mode 100644
index 000000000..badf38de0
--- /dev/null
+++ b/src/main/java/dk/camelot64/kickc/passes/Pass1AssertUsedVars.java
@@ -0,0 +1,78 @@
+package dk.camelot64.kickc.passes;
+
+import dk.camelot64.kickc.model.*;
+
+import java.util.Collection;
+import java.util.LinkedHashSet;
+
+/**
+ * Checks that all used variables are assigned a value before their use
+ */
+public class Pass1AssertUsedVars extends Pass1Base {
+
+   public Pass1AssertUsedVars(Program program) {
+      super(program);
+   }
+
+   @Override
+   public boolean executeStep() {
+
+      new Pass3StatementIndices(getProgram()).generateStatementIndices();
+      new Pass3VariableReferenceInfos(getProgram()).generateVariableReferenceInfos();
+      VariableReferenceInfos referenceInfos = getProgram().getVariableReferenceInfos();
+
+      ControlFlowBlock beginBlock = getProgram().getGraph().getBlock(new LabelRef(SymbolRef.BEGIN_BLOCK_NAME));
+      assertUsedVars(beginBlock, referenceInfos, new LinkedHashSet<>(), new LinkedHashSet<>());
+      getProgram().setVariableReferenceInfos(null);
+      new Pass3StatementIndices(getProgram()).clearStatementIndices();
+
+      return false;
+   }
+
+
+   /**
+    * Assert that all used vars have been assigned values before the use.
+    * Follow the control flow of the graph recursively.
+    * @param block The block to examine
+    * @param referenceInfos Information about assigned/used variables in statements
+    * @param defined Variables already assigned a value at the point of the first execution of the block
+    * @param visited Blocks already visited
+    */
+   public void assertUsedVars(ControlFlowBlock block, VariableReferenceInfos referenceInfos, Collection<VariableRef> defined, Collection<LabelRef> visited) {
+      if(visited.contains(block.getLabel())) {
+         return;
+      }
+      visited.add(block.getLabel());
+      for (Statement statement : block.getStatements()) {
+         Collection<VariableRef> used =  referenceInfos.getUsed(statement);
+         for (VariableRef usedRef : used) {
+            if(!defined.contains(usedRef)) {
+               throw new CompileError("Error! Variable used before being defined "+usedRef.toString(getProgram())+" in "+statement.toString(getProgram(), false));
+            }
+         }
+         Collection<VariableRef> defd = referenceInfos.getDefined(statement);
+         for (VariableRef definedRef : defd) {
+            defined.add(definedRef);
+         }
+         if(statement instanceof StatementCall) {
+            StatementCall call = (StatementCall) statement;
+            Procedure procedure = getProgram().getScope().getProcedure(call.getProcedure());
+            for (String paramName : procedure.getParameterNames()) {
+               defined.add(procedure.getVariable(paramName).getRef());
+            }
+            ControlFlowBlock procedureStart = getProgram().getGraph().getBlock(call.getProcedure().getLabelRef());
+
+            assertUsedVars(procedureStart, referenceInfos, defined, visited);
+         } else if(statement instanceof StatementConditionalJump) {
+            StatementConditionalJump cond= (StatementConditionalJump) statement;
+            ControlFlowBlock jumpTo  = getProgram().getGraph().getBlock(cond.getDestination());
+            assertUsedVars(jumpTo, referenceInfos, defined, visited);
+         }
+      }
+      ControlFlowBlock successor  = getProgram().getGraph().getBlock(block.getDefaultSuccessor());
+      if(successor!=null) {
+         assertUsedVars(successor, referenceInfos, defined, visited);
+      }
+   }
+
+}
diff --git a/src/main/java/dk/camelot64/kickc/passes/Pass1EliminateUnusedVars.java b/src/main/java/dk/camelot64/kickc/passes/Pass1EliminateUnusedVars.java
index 486dcc362..59dcd12df 100644
--- a/src/main/java/dk/camelot64/kickc/passes/Pass1EliminateUnusedVars.java
+++ b/src/main/java/dk/camelot64/kickc/passes/Pass1EliminateUnusedVars.java
@@ -17,7 +17,6 @@ public class Pass1EliminateUnusedVars extends Pass1Base {
    public boolean executeStep() {
       new Pass3StatementIndices(getProgram()).generateStatementIndices();
       new Pass3VariableReferenceInfos(getProgram()).generateVariableReferenceInfos();
-
       VariableReferenceInfos referenceInfos = getProgram().getVariableReferenceInfos();
 
       boolean modified = false;
@@ -51,7 +50,6 @@ public class Pass1EliminateUnusedVars extends Pass1Base {
 
       getProgram().setVariableReferenceInfos(null);
       new Pass3StatementIndices(getProgram()).clearStatementIndices();
-
       return modified;
    }
 
diff --git a/src/main/java/dk/camelot64/kickc/test/TestErrors.java b/src/main/java/dk/camelot64/kickc/test/TestErrors.java
index 0744a982b..113c29971 100644
--- a/src/main/java/dk/camelot64/kickc/test/TestErrors.java
+++ b/src/main/java/dk/camelot64/kickc/test/TestErrors.java
@@ -32,11 +32,6 @@ public class TestErrors extends TestCase {
       compileAndCompare("inline-asm-param");
    }
 
-   public void testUseUninitialized() throws IOException, URISyntaxException {
-      String filename = "useuninitialized";
-      compileAndCompare(filename);
-   }
-
    public void testForRangeSymbolic() throws IOException, URISyntaxException {
       String filename = "forrangesymbolic";
       compileAndCompare(filename);
diff --git a/src/main/java/dk/camelot64/kickc/test/TestPrograms.java b/src/main/java/dk/camelot64/kickc/test/TestPrograms.java
index 3d6f0c65e..cffa8149e 100644
--- a/src/main/java/dk/camelot64/kickc/test/TestPrograms.java
+++ b/src/main/java/dk/camelot64/kickc/test/TestPrograms.java
@@ -287,6 +287,16 @@ public class TestPrograms extends TestCase {
       fail("Expected compile error.");
    }
 
+   public void testUseUninitialized() throws IOException, URISyntaxException {
+      try {
+         compileAndCompare("useuninitialized");
+      } catch (CompileError e) {
+         // expecting error!
+         return;
+      }
+      fail("Expected compile error.");
+   }
+
    public void testTypeMismatch() throws IOException, URISyntaxException {
       try {
          compileAndCompare("typemismatch");
diff --git a/src/main/java/dk/camelot64/kickc/test/useuninitialized.kc b/src/main/java/dk/camelot64/kickc/test/useuninitialized.kc
index 71c713e9a..61c7c8696 100644
--- a/src/main/java/dk/camelot64/kickc/test/useuninitialized.kc
+++ b/src/main/java/dk/camelot64/kickc/test/useuninitialized.kc
@@ -1,7 +1,13 @@
 // Use an uninitialized variable - should fail gracefully!
 // Currently it gets into the optimization phase, where the optimization runs into problems understanding the assignment/usage-graph and ends up failing on an exception
+byte b;
+byte s;
+byte w=2;
 
 void main() {
-   byte b;
-   byte s=b+1;
+   s=b+1;
+   b=3;
+   byte* screen = $0400;
+   *screen = b;
+   *(screen+1) = s;
 }
\ No newline at end of file