diff --git a/_imaging.c b/_imaging.c
index 730f132ed..2c60407d2 100644
--- a/_imaging.c
+++ b/_imaging.c
@@ -1819,7 +1819,7 @@ _box_blur(ImagingObject* self, PyObject* args)
     if (!imOut)
         return NULL;
 
-    if (!ImagingBoxBlur(imIn, imOut, radius, n))
+    if (!ImagingBoxBlur(imOut, imIn, radius, n))
         return NULL;
 
     return PyImagingNew(imOut);
diff --git a/libImaging/BoxBlur.c b/libImaging/BoxBlur.c
index 8be9c77d3..a0e23c4cd 100644
--- a/libImaging/BoxBlur.c
+++ b/libImaging/BoxBlur.c
@@ -232,7 +232,7 @@ HorizontalBoxBlur(Imaging im, Imaging imOut, float floatRadius)
 
 
 Imaging
-ImagingBoxBlur(Imaging im, Imaging imOut, float radius, int n)
+ImagingBoxBlur(Imaging imOut, Imaging imIn, float radius, int n)
 {
     int i;
 
@@ -242,32 +242,32 @@ ImagingBoxBlur(Imaging im, Imaging imOut, float radius, int n)
         );
     }
 
-    if (strcmp(im->mode, imOut->mode) ||
-        im->type  != imOut->type  ||
-        im->bands != imOut->bands ||
-        im->xsize != imOut->xsize ||
-        im->ysize != imOut->ysize)
+    if (strcmp(imIn->mode, imOut->mode) ||
+        imIn->type  != imOut->type  ||
+        imIn->bands != imOut->bands ||
+        imIn->xsize != imOut->xsize ||
+        imIn->ysize != imOut->ysize)
         return ImagingError_Mismatch();
 
-    if (im->type != IMAGING_TYPE_UINT8)
+    if (imIn->type != IMAGING_TYPE_UINT8)
         return ImagingError_ModeError();
 
-    if ( ! (strcmp(im->mode, "RGB") == 0 ||
-            strcmp(im->mode, "RGBA") == 0 ||
-            strcmp(im->mode, "RGBX") == 0 ||
-            strcmp(im->mode, "CMYK") == 0 ||
-            strcmp(im->mode, "L") == 0 ||
-            strcmp(im->mode, "LA") == 0))
+    if ( ! (strcmp(imIn->mode, "RGB") == 0 ||
+            strcmp(imIn->mode, "RGBA") == 0 ||
+            strcmp(imIn->mode, "RGBX") == 0 ||
+            strcmp(imIn->mode, "CMYK") == 0 ||
+            strcmp(imIn->mode, "L") == 0 ||
+            strcmp(imIn->mode, "LA") == 0))
         return ImagingError_ModeError();
 
-    /* Create transposed temp image (im->ysize x im->xsize). */
-    Imaging temp = ImagingNew(im->mode, im->ysize, im->xsize);
+    /* Create transposed temp image (imIn->ysize x imIn->xsize). */
+    Imaging temp = ImagingNew(imIn->mode, imIn->ysize, imIn->xsize);
     if ( ! temp)
         return NULL;
 
     /* Apply one-dimensional blur.
        HorizontalBoxBlur transposes image at same time. */
-    HorizontalBoxBlur(im, imOut, radius);
+    HorizontalBoxBlur(imIn, imOut, radius);
     for (i = 1; i < n; i ++) {
         HorizontalBoxBlur(imOut, imOut, radius);
     }
diff --git a/libImaging/Imaging.h b/libImaging/Imaging.h
index 8b3ce1b81..130975982 100644
--- a/libImaging/Imaging.h
+++ b/libImaging/Imaging.h
@@ -309,7 +309,7 @@ extern Imaging ImagingTransform(
     int fill);
 extern Imaging ImagingUnsharpMask(
     Imaging im, Imaging imOut, float radius, int percent, int threshold);
-extern Imaging ImagingBoxBlur(Imaging im, Imaging imOut, float radius, int n);
+extern Imaging ImagingBoxBlur(Imaging imOut, Imaging imIn, float radius, int n);
 
 extern Imaging ImagingCopy2(Imaging imOut, Imaging imIn);
 extern Imaging ImagingConvert2(Imaging imOut, Imaging imIn);