diff --git a/flang/include/flang/Optimizer/Dialect/FIRTypes.td b/flang/include/flang/Optimizer/Dialect/FIRTypes.td
--- a/flang/include/flang/Optimizer/Dialect/FIRTypes.td
+++ b/flang/include/flang/Optimizer/Dialect/FIRTypes.td
@@ -492,7 +492,7 @@
     bool hasUnknownShape() const { return getShape().empty(); }
 
     // The value `-1` represents an unknown extent for a dimension
-    static constexpr Extent getUnknownExtent() { return -1; }
+    static constexpr Extent getUnknownExtent() { return ShapedType::kDynamicSize; }
   }];
 }