diff --git a/src/libImaging/ImagingSIMD.h b/src/libImaging/ImagingSIMD.h
index d082837d1..35a0fccfe 100644
--- a/src/libImaging/ImagingSIMD.h
+++ b/src/libImaging/ImagingSIMD.h
@@ -1,13 +1,18 @@
+#ifndef __IMAGING_SIMD_H__
+#define __IMAGING_SIMD_H__
+
 /* Microsoft compiler doesn't limit intrinsics for an architecture.
    This macro is set only on x86 and means SSE2 and above including AVX2. */
 #if defined(_M_X64) || _M_IX86_FP == 2
     #define __SSE4_2__
 #endif
+
 #if defined(__SSE4_2__)
     #include <emmintrin.h>
     #include <mmintrin.h>
     #include <smmintrin.h>
 #endif
+
 #if defined(__AVX2__)
     #include <immintrin.h>
 #endif
@@ -25,3 +30,5 @@ mm256_cvtepu8_epi32(void *ptr) {
     return _mm256_cvtepu8_epi32(_mm_loadl_epi64((__m128i *) ptr));
 }
 #endif
+
+#endif