diff --git a/sigv4.c b/sigv4.c
index d01f944..ed93a58 100644
--- a/sigv4.c
+++ b/sigv4.c
@@ -274,7 +274,7 @@ static void get_signed_headers(aws_sigv4_params_t *sigv4_params,
   for (i = 0; i < sigv4_params->num_headers && num_headers < AWS_SIGV4_MAX_NUM_HEADERS; i++)
   {
     // check if the header is x-amz-content-sha256
-    if (strncasecmp(sigv4_params->headers[i].key.data, "x-amz-content-sha256", sigv4_params->headers[i].key.len) == 0)
+    if (strncasecmp((char *)sigv4_params->headers[i].key.data, "x-amz-content-sha256", sigv4_params->headers[i].key.len) == 0)
     {
       has_amz_content_sha256_header = true;
     }
@@ -323,7 +323,7 @@ static void get_canonical_headers(aws_sigv4_params_t *sigv4_params,
   for (i = 0; i < sigv4_params->num_headers && num_headers < AWS_SIGV4_MAX_NUM_HEADERS; i++)
   {
     // check if the header is x-amz-content-sha256
-    if (strncasecmp(sigv4_params->headers[i].key.data, "x-amz-content-sha256", sigv4_params->headers[i].key.len) == 0)
+    if (strncasecmp((char *)sigv4_params->headers[i].key.data, "x-amz-content-sha256", sigv4_params->headers[i].key.len) == 0)
     {
       amz_content_sha256_header = &sigv4_params->headers[i];
     }
diff --git a/sigv4.h b/sigv4.h
index 177ce3b..19ed6a2 100644
--- a/sigv4.h
+++ b/sigv4.h
@@ -13,6 +13,7 @@
 #include <stdbool.h>
 #include <stdint.h>
 #include <string.h>
+#include <strings.h> /* strncasecmp; not pulled in by string.h under -std=c99 */
 #include <ctype.h>
 #endif