diff --git a/changes/bug21894_029 b/changes/bug21894_029
new file mode 100644
index 000000000..e3a84fa72
--- /dev/null
+++ b/changes/bug21894_029
@@ -0,0 +1,5 @@
+  o Minor bugfixes (crash prevention):
+    - Fix an (currently untriggerable, but potentially dangerous) crash
+      bug when base32-encoding inputs whose sizes are not a multiple of
+      5. Fixes bug 21894; bugfix on 0.2.9.1-alpha.
+
diff --git a/src/common/util_format.c b/src/common/util_format.c
index 6e0a04586..7e8ee1b86 100644
--- a/src/common/util_format.c
+++ b/src/common/util_format.c
@@ -51,9 +51,10 @@ base32_encode(char *dest, size_t destlen, const char *src, size_t srclen)
 
   for (i=0,bit=0; bit < nbits; ++i, bit+=5) {
     /* set v to the 16-bit value starting at src[bits/8], 0-padded. */
-    v = ((uint8_t)src[bit/8]) << 8;
-    if (bit+5<nbits)
-      v += (uint8_t)src[(bit/8)+1];
+    size_t idx = bit / 8;
+    v = ((uint8_t)src[idx]) << 8;
+    if (idx+1 < srclen)
+      v += (uint8_t)src[idx+1];
     /* set u to the 5-bit value at the bit'th bit of buf. */
     u = (v >> (11-(bit%8))) & 0x1F;
     dest[i] = BASE32_CHARS[u];