diff --git a/src/random.c b/src/random.c
index 202a4eb..10075ee 100644
--- a/src/random.c
+++ b/src/random.c
@@ -46,7 +46,7 @@
 #define	LOGGAMMA(x)	lgamma_r(x,&signp)	/* Generally slower */
 #else
 #define	LOGGAMMA	gammln
-static double		gammln();
+static double		gammln(double x);
 #endif
 
 #ifdef USE_THREADS