diff --git a/libavcodec/x86/idctdsp.h b/libavcodec/x86/idctdsp.h
index 6408df1e32..daa4e798ed 100644
--- a/libavcodec/x86/idctdsp.h
+++ b/libavcodec/x86/idctdsp.h
@@ -20,6 +20,7 @@
 #define AVCODEC_X86_IDCTDSP_H
 
 #include <stdint.h>
+#include <stddef.h>
 
 void ff_add_pixels_clamped_mmx(const int16_t *block, uint8_t *pixels,
                                ptrdiff_t line_size);