diff --git a/library/poly1305.c b/library/poly1305.c
index bafe613..e22d3af 100644
--- a/library/poly1305.c
+++ b/library/poly1305.c
@@ -44,6 +44,11 @@
 
 #if !defined(MBEDTLS_POLY1305_ALT)
 
+#if ( defined(__ARMCC_VERSION) || defined(_MSC_VER) ) && \
+    !defined(inline) && !defined(__cplusplus)
+#define inline __inline
+#endif
+
 #define POLY1305_BLOCK_SIZE_BYTES ( 16U )
 
 #define BYTES_TO_U32_LE( data, offset )                           \