diff --git a/test/infcover.c b/test/infcover.c
index 6a9999e0..e436888b 100644
--- a/test/infcover.c
+++ b/test/infcover.c
@@ -626,7 +626,7 @@ static void cover_trees(void) {
     code *next, table[ENOUGH_DISTS];
 
     /* we need to call inflate_table() directly in order to manifest not-
-       enough errors, since zlib insures that enough is always enough */
+       enough errors, since zlib ensures that enough is always enough */
     for (bits = 0; bits < 15; bits++)
         lens[bits] = (uint16_t)(bits + 1);
     lens[15] = 15;
diff --git a/zlib-ng.h.in b/zlib-ng.h.in
index 5a8c04f0..527df057 100644
--- a/zlib-ng.h.in
+++ b/zlib-ng.h.in
@@ -880,7 +880,7 @@ int32_t zng_inflateSetDictionary(zng_stream *strm, const uint8_t *dictionary, ui
    deflateSetDictionary).  For raw inflate, this function can be called at any
    time to set the dictionary.  If the provided dictionary is smaller than the
    window and there is already data in the window, then the provided dictionary
-   will amend what's there.  The application must insure that the dictionary
+   will amend what's there.  The application must ensure that the dictionary
    that was used for compression is provided.
 
      inflateSetDictionary returns Z_OK if success, Z_STREAM_ERROR if a
diff --git a/zlib.h.in b/zlib.h.in
index 1dc4f7ec..3bcbca20 100644
--- a/zlib.h.in
+++ b/zlib.h.in
@@ -890,7 +890,7 @@ Z_EXTERN int Z_EXPORT inflateSetDictionary(z_stream *strm, const unsigned char *
    deflateSetDictionary).  For raw inflate, this function can be called at any
    time to set the dictionary.  If the provided dictionary is smaller than the
    window and there is already data in the window, then the provided dictionary
-   will amend what's there.  The application must insure that the dictionary
+   will amend what's there.  The application must ensure that the dictionary
    that was used for compression is provided.
 
      inflateSetDictionary returns Z_OK if success, Z_STREAM_ERROR if a