diff --git a/flang/lib/Optimizer/Builder/FIRBuilder.cpp b/flang/lib/Optimizer/Builder/FIRBuilder.cpp
--- a/flang/lib/Optimizer/Builder/FIRBuilder.cpp
+++ b/flang/lib/Optimizer/Builder/FIRBuilder.cpp
@@ -1234,7 +1234,8 @@
       [&](mlir::Value value) -> llvm::Optional<std::int64_t> {
     if (auto valInt = fir::factory::getIntIfConstant(value))
       return valInt;
-    else if (auto valOp = mlir::dyn_cast<fir::ConvertOp>(value.getDefiningOp()))
+    else if (auto valOp =
+                 mlir::dyn_cast_or_null<fir::ConvertOp>(value.getDefiningOp()))
       return getConstantValue(valOp.getValue());
     return {};
   };
diff --git a/flang/test/Lower/getExtentFromTripletFix.f90 b/flang/test/Lower/getExtentFromTripletFix.f90
new file mode 100644
--- /dev/null
+++ b/flang/test/Lower/getExtentFromTripletFix.f90
@@ -0,0 +1,16 @@
+! Test that the get_extent_fix function works without crashing
+! RUN: bbc -emit-fir %s -o - | FileCheck %s
+! RUN: %flang_fc1 -emit-fir %s -o - | FileCheck %s
+
+
+! CHECK-LABEL: func @_QPget_extent_fix
+! CHECK: fir.do_loop
+! CHECK: fir.call @_FortranAProductInteger
+! CHECK: fir.do_loop
+! CHECK: fir.array_merge_store
+
+subroutine get_extent_fix(map)
+  integer, dimension(:) :: map
+  integer :: counter
+  map = (/ (product(map(:counter)), counter = 1, sizeof(map)) /)
+end subroutine