diff --git a/src/libutil/util.cc b/src/libutil/util.cc
index 5bb3f374ba7..27f5dcfecd9 100644
--- a/src/libutil/util.cc
+++ b/src/libutil/util.cc
@@ -200,8 +200,9 @@ std::string base64Decode(std::string_view s)
         if (c == '\n') continue;
 
         char digit = base64DecodeChars[(unsigned char) c];
-        if (digit == npos)
-            throw Error("invalid character in Base64 string: '%c'", c);
+        if (digit == npos) {
+            throw Error("invalid character in Base64 string: '%c' in '%s'", c, s.data());
+        }
 
         bits += 6;
         d = d << 6 | digit;