diff --git a/src/core/support/string.h b/src/core/support/string.h
index 2ce4b7f7856bcfc64c18f5cac01fc176433bc2f4..28b7029ecd65c77d36311b8c65ae35e2620ff31c 100644
--- a/src/core/support/string.h
+++ b/src/core/support/string.h
@@ -64,7 +64,8 @@ int gpr_parse_bytes_to_uint32(const char *data, size_t length,
 #define GPR_LTOA_MIN_BUFSIZE (3 * sizeof(long))
 
 /* Convert a long to a string in base 10; returns the length of the
-   output string (or 0 on failure) */
+   output string (or 0 on failure).
+   output must be at least GPR_LTOA_MIN_BUFSIZE bytes long. */
 int gpr_ltoa(long value, char *output);
 
 /* Reverse a run of bytes */