diff --git a/detectron2/modeling/postprocessing.py b/detectron2/modeling/postprocessing.py
index a915491e32..52f273bb5e 100644
--- a/detectron2/modeling/postprocessing.py
+++ b/detectron2/modeling/postprocessing.py
@@ -27,7 +27,8 @@ def detector_postprocess(
     Returns:
         Instances: the resized output from the model, based on the output resolution
     """
-    if torch.jit.is_tracing():
+    if isinstance(output_width, torch.Tensor):
+        # This shape might (but not necessarily) be tensors during tracing.
         # Converts integer tensors to float temporaries to ensure true
         # division is performed when computing scale_x and scale_y.
         output_width_tmp = output_width.float()
diff --git a/tests/test_model_analysis.py b/tests/test_model_analysis.py
index deee4fa78d..234ec8f561 100644
--- a/tests/test_model_analysis.py
+++ b/tests/test_model_analysis.py
@@ -39,6 +39,11 @@ def test_flop(self):
         # almost 0 for random inputs.
         self.assertTrue(int(res["conv"]), 117)
 
+    def test_flop_with_output_shape(self):
+        inputs = [{"image": torch.rand(3, 800, 800), "height": 700, "width": 700}]
+        res = flop_count_operators(self.model, inputs)
+        self.assertTrue(int(res["conv"]), 117)
+
     def test_param_count(self):
         res = parameter_count(self.model)
         self.assertTrue(res[""], 41699936)