diff --git a/Modules/_io/bytesio.c b/Modules/_io/bytesio.c
--- a/Modules/_io/bytesio.c
+++ b/Modules/_io/bytesio.c
@@ -53,7 +53,7 @@
         Py_ssize_t copy_size;
         char *new_buf;
 
-        if((! truncate) && preferred_size < self->string_size) {
+        if((! truncate) && preferred_size < (size_t)self->string_size) {
             preferred_size = self->string_size;
         }
 
@@ -64,7 +64,7 @@
         }
 
         copy_size = self->string_size;
-        if (copy_size > preferred_size) {
+        if ((size_t)copy_size > preferred_size) {
             copy_size = preferred_size;
         }
 
@@ -620,7 +620,7 @@
         return NULL;
     }
 
-    if (unshare(self, size, 1) < 0) {
+    if (unshare(self, (size_t)size, 1) < 0) {
         return NULL;
     }