diff --git a/flang/lib/Optimizer/Builder/IntrinsicCall.cpp b/flang/lib/Optimizer/Builder/IntrinsicCall.cpp
--- a/flang/lib/Optimizer/Builder/IntrinsicCall.cpp
+++ b/flang/lib/Optimizer/Builder/IntrinsicCall.cpp
@@ -3925,14 +3925,29 @@
   mlir::Type type0 = fir::unwrapRefType(tsource.getType());
   bool isCharRslt = fir::isa_char(type0); // result is same as first argument
   mlir::Value mask = builder.createConvert(loc, builder.getI1Type(), rawMask);
-  // FSOURCE has the same type as TSOURCE, but they may not have the same MLIR
-  // types (one can have dynamic length while the other has constant lengths,
-  // or one may be a fir.logical<> while the other is an i1). Insert a cast to
-  // fulfill mlir::SelectOp constraint that the MLIR types must be the same.
-  mlir::Value fsourceCast =
-      builder.createConvert(loc, tsource.getType(), fsource);
-  auto rslt =
-      builder.create<mlir::arith::SelectOp>(loc, mask, tsource, fsourceCast);
+
+  // The result is polymorphic if and only if both TSOURCE and FSOURCE are
+  // polymorphic. TSOURCE and FSOURCE are required to have the same type
+  // (for both declared and dynamic types) so a simple convert op can be
+  // used.
+  mlir::Value tsourceCast = tsource;
+  mlir::Value fsourceCast = fsource;
+  if (fir::isPolymorphicType(tsource.getType()) &&
+      !fir::isPolymorphicType(fsource.getType())) {
+    tsourceCast = builder.createConvert(loc, fsource.getType(), tsource);
+  } else if (!fir::isPolymorphicType(tsource.getType()) &&
+             fir::isPolymorphicType(fsource.getType())) {
+    fsourceCast = builder.createConvert(loc, tsource.getType(), fsource);
+  } else {
+    // FSOURCE and TSOURCE are not polymorphic.
+    // FSOURCE has the same type as TSOURCE, but they may not have the same MLIR
+    // types (one can have dynamic length while the other has constant lengths,
+    // or one may be a fir.logical<> while the other is an i1). Insert a cast to
+    // fulfill mlir::SelectOp constraint that the MLIR types must be the same.
+    fsourceCast = builder.createConvert(loc, tsource.getType(), fsource);
+  }
+  auto rslt = builder.create<mlir::arith::SelectOp>(loc, mask, tsourceCast,
+                                                    fsourceCast);
   if (isCharRslt) {
     // Need a CharBoxValue for character results
     const fir::CharBoxValue *charBox = args[0].getCharBox();
diff --git a/flang/test/Lower/polymorphic-temp.f90 b/flang/test/Lower/polymorphic-temp.f90
--- a/flang/test/Lower/polymorphic-temp.f90
+++ b/flang/test/Lower/polymorphic-temp.f90
@@ -215,7 +215,6 @@
     call check_scalar(merge(a, b, i==1))
   end subroutine
 
-
 ! CHECK-LABEL: func.func @_QMpoly_tmpPtest_merge_intrinsic2(
 ! CHECK-SAME: %[[A:.*]]: !fir.ref<!fir.class<!fir.heap<!fir.type<_QMpoly_tmpTp1{a:i32}>>>> {fir.bindc_name = "a"}, %[[B:.*]]: !fir.ref<!fir.box<!fir.heap<!fir.type<_QMpoly_tmpTp1{a:i32}>>>> {fir.bindc_name = "b"}, %[[I:.*]]: !fir.ref<i32> {fir.bindc_name = "i"}) {
 ! CHECK: %[[LOAD_A:.*]] = fir.load %[[A]] : !fir.ref<!fir.class<!fir.heap<!fir.type<_QMpoly_tmpTp1{a:i32}>>>>
@@ -223,7 +222,7 @@
 ! CHECK: %[[LOAD_I:.*]] = fir.load %[[I]] : !fir.ref<i32>
 ! CHECK: %[[C1:.*]] = arith.constant 1 : i32
 ! CHECK: %[[CMPI:.*]] = arith.cmpi eq, %[[LOAD_I]], %[[C1]] : i32
-! CHECK: %[[B_CONV:.*]] = fir.convert %[[LOAD_B]] : (!fir.box<!fir.heap<!fir.type<_QMpoly_tmpTp1{a:i32}>>>) -> !fir.class<!fir.heap<!fir.type<_QMpoly_tmpTp1{a:i32}>>>
-! CHECK: %{{.*}} = arith.select %[[CMPI]], %[[LOAD_A]], %[[B_CONV]] : !fir.class<!fir.heap<!fir.type<_QMpoly_tmpTp1{a:i32}>>>
+! CHECK: %[[A_CONV:.*]] = fir.convert %[[LOAD_A]] : (!fir.class<!fir.heap<!fir.type<_QMpoly_tmpTp1{a:i32}>>>) -> !fir.box<!fir.heap<!fir.type<_QMpoly_tmpTp1{a:i32}>>>
+! CHECK: %{{.*}} = arith.select %[[CMPI]], %[[A_CONV]], %[[LOAD_B]] : !fir.box<!fir.heap<!fir.type<_QMpoly_tmpTp1{a:i32}>>>
 
 end module