diff --git a/compiler/optimizer/OMRValuePropagation.cpp b/compiler/optimizer/OMRValuePropagation.cpp
index c8bf0c21f38..ebe39bbcd9e 100644
--- a/compiler/optimizer/OMRValuePropagation.cpp
+++ b/compiler/optimizer/OMRValuePropagation.cpp
@@ -4091,14 +4091,15 @@ void TR::GlobalValuePropagation::processStructure(TR_StructureSubGraphNode *node
          }
       else
          {
-         // Only commoned nodes within an extended block need to be set
-         // in _defMergedNodes. So it's safe to reset it here
-         _defMergedNodes->empty();
          processImproperLoop(node, lastTimeThrough, insideLoop);
          }
       }
    else
       {
+      // Only commoned nodes within a block need to be set in _defMergedNodes.
+      // So it should be reset here
+      TR_ASSERT_FATAL(!node->getStructure()->asBlock()->getBlock()->isExtensionOfPreviousBlock(), "This optimization does not run on extended blocks");
+      _defMergedNodes->empty();
       processBlock(node, lastTimeThrough, insideLoop);
       }
    }
@@ -4469,10 +4470,6 @@ void TR::GlobalValuePropagation::processRegionSubgraph(TR_StructureSubGraphNode
    // Current constraints have already been set up to be the input constraints.
    //
    
-   // Only commoned nodes within an extended block need to be set
-   // in _defMergedNodes. So it's safe to reset it here
-   _defMergedNodes->empty();
-
    TR_RegionStructure *region = node->getStructure()->asRegion();
    TR_StructureSubGraphNode *entry = region->getEntry();