diff --git a/modules/png/core/common.scm b/modules/png/core/common.scm
index 5cc0cfc..eb4552d 100644
--- a/modules/png/core/common.scm
+++ b/modules/png/core/common.scm
@@ -33,6 +33,7 @@
             int16->bytevector
             int32->bytevector
             bytevector-copy/part
+            bytevector-copy/part!
             bytevector-split
             constructor-argument
             re-export-modules))
@@ -71,6 +72,14 @@
     (bytevector-copy! bv source-start result 0 len)
     result))
 
+(define* (bytevector-copy/part! bv
+                                result
+                                #:key
+                                (source-offset 0)
+                                (source-length (bytevector-length bv))
+                                (target-offset 0))
+  (bytevector-copy! bv source-offset result target-offset source-length))
+
 (define-method (bytevector-split (bv <bytevector>) (chunk-size <number>))
   "Split a bytevector BV into parts of CHUNK-SIZE.  Return a list of
 bytevectors."