diff -ur rxvt-unicode-9.22-a/src/rxvtfont.C rxvt-unicode-9.22-b/src/rxvtfont.C
--- rxvt-unicode-9.22-a/src/rxvtfont.C	2021-01-26 13:41:32.889882053 +0100
+++ rxvt-unicode-9.22-b/src/rxvtfont.C	2021-01-26 13:43:25.909883427 +0100
@@ -155,7 +155,7 @@
 // these characters are used to guess the font height and width
 // pango uses a similar algorithm and doesn't trust the font either.
 static uint16_t extent_test_chars[] = {
-  '0', '1', '8', 'a', 'd', 'x', 'm', 'y', 'g', 'W', 'X', '\'', '_',
+  ' ', '0', '1', '8', 'a', 'd', 'x', 'm', 'y', 'g', 'W', 'X', '\'', '_',
   0x00cd, 0x00d5, 0x0114, 0x0177, 0x0643,	// ÍÕĔŷﻙ
   0x304c, 0x672c,				// が本
 };