@@ -1653,6 +1653,9 @@ def imsave(fname, arr, vmin=None, vmax=None, cmap=None, format=None,
             rgba = sm.to_rgba(arr, bytes=True)
         if pil_kwargs is None:
             pil_kwargs = {}
+        else:
+            # we modify this below, so make a copy (don't modify caller's dict)
+            pil_kwargs = pil_kwargs.copy()
         pil_shape = (rgba.shape[1], rgba.shape[0])
         image = PIL.Image.frombuffer(
             "RGBA", pil_shape, rgba, "raw", "RGBA", 0, 1)
 
@@ -254,9 +254,11 @@ def test_pil_kwargs_webp():
     buf_small = io.BytesIO()
     pil_kwargs_low = {"quality": 1}
     plt.savefig(buf_small, format="webp", pil_kwargs=pil_kwargs_low)
+    assert len(pil_kwargs_low) == 1
     buf_large = io.BytesIO()
     pil_kwargs_high = {"quality": 100}
     plt.savefig(buf_large, format="webp", pil_kwargs=pil_kwargs_high)
+    assert len(pil_kwargs_high) == 1
     assert buf_large.getbuffer().nbytes > buf_small.getbuffer().nbytes
 
 
 
@@ -249,6 +249,7 @@ def test_imsave_pil_kwargs_tiff():
     buf = io.BytesIO()
     pil_kwargs = {"description": "test image"}
     plt.imsave(buf, [[0, 1], [2, 3]], format="tiff", pil_kwargs=pil_kwargs)
+    assert len(pil_kwargs) == 1
     im = Image.open(buf)
     tags = {TAGS[k].name: v for k, v in im.tag_v2.items()}
     assert tags["ImageDescription"] == "test image"