diff --git a/src/nmod_poly.h b/src/nmod_poly.h
index 16bd376bbb..cb9a889f5c 100644
--- a/src/nmod_poly.h
+++ b/src/nmod_poly.h
@@ -206,7 +206,7 @@ void nmod_poly_one(nmod_poly_t res)
     res->coeffs[0] = 1;
 }
 
-void nmod_poly_set_trunc(nmod_poly_t res, const nmod_poly_t poly, slong n);
+void nmod_poly_set_trunc(nmod_poly_t res, const nmod_poly_t poly, slong len);
 
 NMOD_POLY_INLINE
 void nmod_poly_truncate(nmod_poly_t poly, slong len)
diff --git a/src/nmod_poly/set_trunc.c b/src/nmod_poly/set_trunc.c
index ae92c04c38..ee220aa950 100644
--- a/src/nmod_poly/set_trunc.c
+++ b/src/nmod_poly/set_trunc.c
@@ -17,10 +17,10 @@ nmod_poly_set_trunc(nmod_poly_t res, const nmod_poly_t poly, slong len)
 {
     if (poly == res)
     {
-        if (poly->length > len)
+        if (res->length > len)
         {
-            poly->length = len;
-            _nmod_poly_normalise(poly);
+            res->length = len;
+            _nmod_poly_normalise(res);
         }
     }
     else