diff --git a/src/libiir/200_iir.c b/src/libiir/200_iir.c
index a730171..d311b67 100644
--- a/src/libiir/200_iir.c
+++ b/src/libiir/200_iir.c
@@ -149,7 +149,8 @@ iir_filter_chain(struct iir_filter_t* fi, const struct iir_coeff_t* coeff)
 /* iir_filter_copy()
  *  Performs a deep copy of a filter instance chain.
  */
-struct iir_filter_t* iir_filter_copy(const struct iir_filter_t* fi, int state)
+struct iir_filter_t*
+iir_filter_copy(const struct iir_filter_t* fi, int state)
 {
     struct iir_filter_t* head = 0, * tail = 0, * copy;
 
@@ -165,7 +166,7 @@ struct iir_filter_t* iir_filter_copy(const struct iir_filter_t* fi, int state)
         memcpy(copy->c, fi->c, sizeof(double) * fi->nc);
         memcpy(copy->d, fi->d, sizeof(double) * fi->nd);
 
-        if(state) {
+        if(state && fi->ready) {
             copy->ready = 1;
             memcpy(copy->x, fi->x, sizeof(double) * fi->nc);
             memcpy(copy->y, fi->y, sizeof(double) * fi->nd);