diff --git a/src/stack.ts b/src/stack.ts
index 3c1209471f..cae7e115f7 100644
--- a/src/stack.ts
+++ b/src/stack.ts
@@ -184,10 +184,10 @@ export function stack(m: Mark | MarkDef, encoding: Encoding<string>): StackPrope
 
     // TODO: Figure out why groupBy is only sometimes necessary?
     // Unclear why the test case and the bug report differ in whether a groupBy should be supplied to vega or not
-    const shouldAddPolarGroupBy = dimensionField === 'dir' || (fieldChannel === dimensionChannel);
+    const shouldAddPolarGroupBy = dimensionField === 'dir' || fieldChannel === dimensionChannel;
 
     // if (!hasSameDimensionAndStackedField) {
-    if (!hasSameDimensionAndStackedField && (isPolar ? (shouldAddPolarGroupBy) : true)) {
+    if (!hasSameDimensionAndStackedField && (isPolar ? shouldAddPolarGroupBy : true)) {
       // avoid grouping by the stacked field
       // TKTK: find out why
       groupbyChannels.push(dimensionChannel);