diff --git a/Documentation/pretty-options.txt b/Documentation/pretty-options.txt
index 27ddaf84a1..42b227bc40 100644
--- a/Documentation/pretty-options.txt
+++ b/Documentation/pretty-options.txt
@@ -40,7 +40,9 @@ people using 80-column terminals.
 	defaults to UTF-8. Note that if an object claims to be encoded
 	in `X` and we are outputting in `X`, we will output the object
 	verbatim; this means that invalid sequences in the original
-	commit may be copied to the output.
+	commit may be copied to the output. Likewise, if iconv(3) fails
+	to convert the commit, we will output the original object
+	verbatim, along with a warning.
 
 --expand-tabs=<n>::
 --expand-tabs::
diff --git a/pretty.c b/pretty.c
index 9631529c10..73b5ead509 100644
--- a/pretty.c
+++ b/pretty.c
@@ -671,7 +671,11 @@ const char *repo_logmsg_reencode(struct repository *r,
 	 * If the re-encoding failed, out might be NULL here; in that
 	 * case we just return the commit message verbatim.
 	 */
-	return out ? out : msg;
+	if (!out) {
+		warning("unable to reencode commit to '%s'", output_encoding);
+		return msg;
+	}
+	return out;
 }
 
 static int mailmap_name(const char **email, size_t *email_len,
diff --git a/t/t4210-log-i18n.sh b/t/t4210-log-i18n.sh
index d2dfcf164e..0141f36e33 100755
--- a/t/t4210-log-i18n.sh
+++ b/t/t4210-log-i18n.sh
@@ -131,4 +131,11 @@ do
 	fi
 done
 
+test_expect_success 'log shows warning when conversion fails' '
+	enc=this-encoding-does-not-exist &&
+	git log -1 --encoding=$enc 2>err &&
+	echo "warning: unable to reencode commit to ${SQ}${enc}${SQ}" >expect &&
+	test_cmp expect err
+'
+
 test_done