diff --git a/src/libImaging/BoxBlur.c b/src/libImaging/BoxBlur.c
index 88862eb73..2e45a3358 100644
--- a/src/libImaging/BoxBlur.c
+++ b/src/libImaging/BoxBlur.c
@@ -287,7 +287,7 @@ ImagingGaussianBlur(Imaging imOut, Imaging imIn, float radius, int passes) {
     float sigma2, L, l, a;
 
     sigma2 = radius * radius / passes;
-    // from http://www.mia.uni-saarland.de/Publications/gwosdek-ssvm11.pdf
+    // from https://www.mia.uni-saarland.de/Publications/gwosdek-ssvm11.pdf
     // [7] Box length.
     L = sqrt(12.0 * sigma2 + 1.0);
     // [11] Integer part of box radius.