diff --git a/mlir/lib/Dialect/SCF/SCF.cpp b/mlir/lib/Dialect/SCF/SCF.cpp
--- a/mlir/lib/Dialect/SCF/SCF.cpp
+++ b/mlir/lib/Dialect/SCF/SCF.cpp
@@ -708,7 +708,9 @@
   } else {
     // If the condition isn't constant, both regions may be executed.
     regions.push_back(RegionSuccessor(&thenRegion()));
-    regions.push_back(RegionSuccessor(elseRegion));
+    // If the else region does not exist, it is not a viable successor.
+    if (elseRegion)
+      regions.push_back(RegionSuccessor(elseRegion));
     return;
   }