diff --git a/src/libiir/200_iir.c b/src/libiir/200_iir.c
index cbf114e..da59edf 100644
--- a/src/libiir/200_iir.c
+++ b/src/libiir/200_iir.c
@@ -22,7 +22,7 @@ struct iir_coeff_t {
  *  Allocates a new set of coefficient objects.
  */
 struct iir_coeff_t*
-iir_coeff_new(int nc, double* c, int nd, double* d)
+iir_coeff_new(int nc, const double* c, int nd, const double* d)
 {
     struct iir_coeff_t* coeff;
 
diff --git a/src/libiir/200_iir.h b/src/libiir/200_iir.h
index a83a2b6..ed6d000 100644
--- a/src/libiir/200_iir.h
+++ b/src/libiir/200_iir.h
@@ -46,7 +46,8 @@ instances through \ref iir_filter().
 See \ref iir_structure for a full explanation of the parameters.
 
 */
-struct iir_coeff_t* iir_coeff_new(int nc, double* c, int nd, double* d)
+struct iir_coeff_t* iir_coeff_new(int nc, const double* c, int nd,
+    const double* d)
 #ifndef DOXYGEN
     __attribute__((malloc,nonnull))
 #endif