diff --git a/ethosu/regor/architecture/ethosu55/ethos_u55_constraints.cpp b/ethosu/regor/architecture/ethosu55/ethos_u55_constraints.cpp
index fd657dcdeca6ed86b55db5f567b2d787edd4961a..090530d9ef08ec58a91eeda3784e5f6d2b2551c0 100644
--- a/ethosu/regor/architecture/ethosu55/ethos_u55_constraints.cpp
+++ b/ethosu/regor/architecture/ethosu55/ethos_u55_constraints.cpp
@@ -281,6 +281,12 @@ bool EthosU55Constraints::SupportedDtypes(OpType opType, DataType ifmType, DataT
 // Validate that zero-points are supported
 bool EthosU55Constraints::SupportedZeroPoint(int64_t zp, TensorUsage usage, DataType dType, OpType opType)
 {
+    if ( !IsSignedInteger(dType) && zp < 0 )
+    {
+        // must be non-negative for unsigned data types
+        return false;
+    }
+
     if ( IsIFM(usage) )
     {
         // must be zero for 32-bit IFM and for CLZ or SHL operations
diff --git a/ethosu/regor/compiler/tflite_graph_optimiser.cpp b/ethosu/regor/compiler/tflite_graph_optimiser.cpp
index 5e722813d24c48e7479b8f84d0bc7f9cffd5afc7..ce1369f434b6fa37ecc07b416ec4c0e4cef1c05e 100644
--- a/ethosu/regor/compiler/tflite_graph_optimiser.cpp
+++ b/ethosu/regor/compiler/tflite_graph_optimiser.cpp
@@ -2648,8 +2648,14 @@ Operation *TFLiteGraphOptimiser::ConvertZeroPoint(Graph *const graph, Operation
 // quantization which may require legalization depending on which hardware is targeted.
 Operation *TFLiteGraphOptimiser::LegalizeAsymmetricQuantization(Graph *const graph, Operation *const operation)
 {
+    UNUSED(graph);
     auto returnOp = operation;
     OpType opType = operation->Type();
+    if ( opType == OpType::Passthrough )
+    {
+        return returnOp;
+    }
+
     TensorConnection *ifmConn = operation->Input(TensorUsage::IFM);
     if ( ifmConn->quantization.zeroPoints.size() == 0 )
     {