diff --git a/src/libespeak-ng/synthdata.c b/src/libespeak-ng/synthdata.c
index 1cfd73ef..32983f0a 100644
--- a/src/libespeak-ng/synthdata.c
+++ b/src/libespeak-ng/synthdata.c
@@ -704,7 +704,7 @@ static void SwitchOnVowelType(PHONEME_LIST *plist, PHONEME_DATA *phdata, unsigne
 	*p_prog += 12;
 }
 
-int NumInstnWords(unsigned short *prog)
+static int NumInstnWords(unsigned short *prog)
 {
 	int instn;
 	int instn2;
diff --git a/src/libespeak-ng/synthdata.h b/src/libespeak-ng/synthdata.h
index 2f2ab3b7..0bc3575b 100644
--- a/src/libespeak-ng/synthdata.h
+++ b/src/libespeak-ng/synthdata.h
@@ -52,7 +52,6 @@ frameref_t *LookupSpect(PHONEME_TAB *this_ph,
 		int *n_frames,
 		PHONEME_LIST *plist);
 
-int NumInstnWords(unsigned short *prog);
 int PhonemeCode(unsigned int mnem);
 int SelectPhonemeTable(int number);
 int  SelectPhonemeTableName(const char *name);