diff --git a/library/pem.c b/library/pem.c
index 33e74ab..7070681 100644
--- a/library/pem.c
+++ b/library/pem.c
@@ -195,12 +195,12 @@
     if( ctx == NULL )
         return( POLARSSL_ERR_PEM_INVALID_DATA );
 
-    s1 = (unsigned char *) strstr( (char *) data, header );
+    s1 = (unsigned char *) strstr( (const char *) data, header );
 
     if( s1 == NULL )
         return( POLARSSL_ERR_PEM_NO_HEADER_PRESENT );
 
-    s2 = (unsigned char *) strstr( (char *) data, footer );
+    s2 = (unsigned char *) strstr( (const char *) data, footer );
 
     if( s2 == NULL || s2 <= s1 )
         return( POLARSSL_ERR_PEM_INVALID_DATA );