diff --git a/libavcodec/synth_filter.h b/libavcodec/synth_filter.h
index 8f1a8b4336..bb5e47cd35 100644
--- a/libavcodec/synth_filter.h
+++ b/libavcodec/synth_filter.h
@@ -18,6 +18,9 @@
  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  */
 
+#ifndef AVCODEC_SYNTH_FILTER_H
+#define AVCODEC_SYNTH_FILTER_H
+
 #include "dsputil.h"
 
 void ff_synth_filter_float(FFTContext *imdct,
@@ -25,3 +28,4 @@ void ff_synth_filter_float(FFTContext *imdct,
                            float synth_buf2[32], const float window[512],
                            float out[32], const float in[32], float scale, float bias);
 
+#endif /* AVCODEC_SYNTH_FILTER_H */