diff --git a/go/vt/sqlparser/normalizer.go b/go/vt/sqlparser/normalizer.go
index dffb90d52b8..dbded43765a 100644
--- a/go/vt/sqlparser/normalizer.go
+++ b/go/vt/sqlparser/normalizer.go
@@ -157,7 +157,10 @@ func newNormalizer(
 // It handles normalization logic based on node types.
 func (nz *normalizer) walkDown(node, _ SQLNode) bool {
 	switch node := node.(type) {
-	case *Begin, *Commit, *Rollback, *Savepoint, *SRollback, *Release, *OtherAdmin, *Analyze, *AssignmentExpr,
+	case *AssignmentExpr:
+		nz.err = vterrors.VT12001("Assignment expression")
+		return false
+	case *Begin, *Commit, *Rollback, *Savepoint, *SRollback, *Release, *OtherAdmin, *Analyze,
 		*PrepareStmt, *ExecuteStmt, *FramePoint, *ColName, TableName, *ConvertType:
 		// These statement don't need normalizing
 		return false
diff --git a/go/vt/sqlparser/parse_test.go b/go/vt/sqlparser/parse_test.go
index 6c4a6e634ec..5bf4d43bca6 100644
--- a/go/vt/sqlparser/parse_test.go
+++ b/go/vt/sqlparser/parse_test.go
@@ -1327,9 +1327,15 @@ var (
 		input: "update /* a.b */ a.b set b = 3",
 	}, {
 		input: "update a.b set d = @v := d + 7 where u = 42",
+		// Assignment expresions aren't supported, so we skip the normalizer test.
+		// We only support parsing these expressions.
+		ignoreNormalizerTest: true,
 	}, {
 		input:  "select @topic3_id:= 10103;",
 		output: "select @topic3_id := 10103 from dual",
+		// Assignment expresions aren't supported, so we skip the normalizer test.
+		// We only support parsing these expressions.
+		ignoreNormalizerTest: true,
 	}, {
 		input: "update /* list */ a set b = 3, c = 4",
 	}, {
diff --git a/go/vt/vtgate/planbuilder/testdata/unsupported_cases.json b/go/vt/vtgate/planbuilder/testdata/unsupported_cases.json
index 55329586b0e..e54050bce86 100644
--- a/go/vt/vtgate/planbuilder/testdata/unsupported_cases.json
+++ b/go/vt/vtgate/planbuilder/testdata/unsupported_cases.json
@@ -229,6 +229,11 @@
     "query": "select @val := 42",
     "plan": "VT12001: unsupported: Assignment expression"
   },
+  {
+    "comment": "Assignment expression in on duplicate clause",
+    "query": "insert into unsharded (id) values (@val := 42)",
+    "plan": "VT12001: unsupported: Assignment expression"
+  },
   {
     "comment": "Assignment expression in union statements",
     "query": "select @val := 42 union select 1",