diff --git a/cpp/ql/lib/semmle/code/cpp/ir/dataflow/internal/DataFlowUtil.qll b/cpp/ql/lib/semmle/code/cpp/ir/dataflow/internal/DataFlowUtil.qll
index 42198d12372..9355493303a 100644
--- a/cpp/ql/lib/semmle/code/cpp/ir/dataflow/internal/DataFlowUtil.qll
+++ b/cpp/ql/lib/semmle/code/cpp/ir/dataflow/internal/DataFlowUtil.qll
@@ -553,9 +553,11 @@ class SsaPhiNode extends Node, TSsaPhiNode {
   cached
   final Node getAnInput(boolean fromBackEdge) {
     localFlowStep(result, this) and
-    if phi.getBasicBlock().dominates(result.getBasicBlock())
-    then fromBackEdge = true
-    else fromBackEdge = false
+    exists(IRBlock bPhi, IRBlock bResult |
+      bPhi = phi.getBasicBlock() and bResult = result.getBasicBlock()
+    |
+      if bPhi.dominates(bResult) then fromBackEdge = true else fromBackEdge = false
+    )
   }
 
   /** Gets a node that is used as input to this phi node. */