diff --git a/tinyexpr.c b/tinyexpr.c
index 05774ff..a5fc9d8 100755
--- a/tinyexpr.c
+++ b/tinyexpr.c
@@ -1,3 +1,4 @@
+// SPDX-License-Identifier: Zlib
 /*
  * TINYEXPR - Tiny recursive descent parser and evaluation engine in C
  *
diff --git a/tinyexpr.h b/tinyexpr.h
index c9afa6c..c2cbe1a 100644
--- a/tinyexpr.h
+++ b/tinyexpr.h
@@ -1,3 +1,4 @@
+// SPDX-License-Identifier: Zlib
 /*
  * TINYEXPR - Tiny recursive descent parser and evaluation engine in C
  *