diff --git a/docs/tracing.txt b/docs/tracing.txt
index 5504850..963c504 100644
--- a/docs/tracing.txt
+++ b/docs/tracing.txt
@@ -74,7 +74,10 @@
 
 Format strings should reflect the types defined in the trace event.  Take
 special care to use PRId64 and PRIu64 for int64_t and uint64_t types,
-respectively.  This ensures portability between 32- and 64-bit platforms.
+respectively.  This ensures portability between 32- and 64-bit platforms.  Note
+that format strings must begin and end with double quotes.  When using
+portability macros, ensure they are preceded and followed by double quotes:
+"value %"PRIx64"".
 
 === Hints for adding new trace events ===