diff --git a/CHANGES b/CHANGES
index afcce93836..8e6d993723 100644
--- a/CHANGES
+++ b/CHANGES
@@ -1,3 +1,6 @@
+5072.	[bug]		Add unit tests for isc_buffer_copyregion() and fix its
+			behavior for auto-reallocated buffers. [GL #644]
+
 5071.	[bug]		Comparision of NXT records was broken. [GL #631]
 
 5070.	[bug]		Record types which support a empty rdata field were
diff --git a/lib/isc/buffer.c b/lib/isc/buffer.c
index 5eb2620c70..987795be12 100644
--- a/lib/isc/buffer.c
+++ b/lib/isc/buffer.c
@@ -514,27 +514,24 @@ isc_buffer_dup(isc_mem_t *mctx, isc_buffer_t **dstp, const isc_buffer_t *src) {
 
 isc_result_t
 isc_buffer_copyregion(isc_buffer_t *b, const isc_region_t *r) {
-	unsigned char *base;
-	unsigned int available;
 	isc_result_t result;
 
 	REQUIRE(ISC_BUFFER_VALID(b));
 	REQUIRE(r != NULL);
 
-	/*
-	 * XXXDCL
-	 */
-	base = isc_buffer_used(b);
-	available = isc_buffer_availablelength(b);
 	if (ISC_UNLIKELY(b->autore)) {
 		result = isc_buffer_reserve(&b, r->length);
-		if (result != ISC_R_SUCCESS)
+		if (result != ISC_R_SUCCESS) {
 			return (result);
+		}
 	}
-	if (r->length > available)
+
+	if (r->length > isc_buffer_availablelength(b)) {
 		return (ISC_R_NOSPACE);
+	}
+
 	if (r->length > 0U) {
-		memmove(base, r->base, r->length);
+		memmove(isc_buffer_used(b), r->base, r->length);
 		b->used += r->length;
 	}
 
diff --git a/lib/isc/tests/buffer_test.c b/lib/isc/tests/buffer_test.c
index 34145d7bcc..89009e0a9d 100644
--- a/lib/isc/tests/buffer_test.c
+++ b/lib/isc/tests/buffer_test.c
@@ -10,17 +10,20 @@
  */
 
 #include <config.h>
+
+#include <limits.h>
+#include <stdbool.h>
 #include <stdlib.h>
-#include <unistd.h>
-#include <fcntl.h>
+#include <string.h>
 
 #include <atf-c.h>
 
 #include "isctest.h"
 
 #include <isc/buffer.h>
-#include <isc/print.h>
+#include <isc/region.h>
 #include <isc/result.h>
+#include <isc/types.h>
 
 ATF_TC(isc_buffer_reserve);
 ATF_TC_HEAD(isc_buffer_reserve, tc) {
@@ -158,6 +161,51 @@ ATF_TC_BODY(isc_buffer_dynamic, tc) {
 	isc_test_end();
 }
 
+ATF_TC(isc_buffer_copyregion);
+ATF_TC_HEAD(isc_buffer_copyregion, tc) {
+	atf_tc_set_md_var(tc, "descr", "copy a region into a buffer");
+}
+
+ATF_TC_BODY(isc_buffer_copyregion, tc) {
+	unsigned char data[] = { 0x11, 0x22, 0x33, 0x44 };
+	isc_buffer_t *b = NULL;
+	isc_result_t result;
+
+	isc_region_t r = {
+		.base = data,
+		.length = sizeof(data),
+	};
+
+	result = isc_test_begin(NULL, true, 0);
+	ATF_REQUIRE_EQ(result, ISC_R_SUCCESS);
+
+	result = isc_buffer_allocate(mctx, &b, sizeof(data));
+	ATF_REQUIRE_EQ(result, ISC_R_SUCCESS);
+
+	/*
+	 * Fill originally allocated buffer space.
+	 */
+	result = isc_buffer_copyregion(b, &r);
+	ATF_CHECK_EQ(result, ISC_R_SUCCESS);
+
+	/*
+	 * Appending more data to the buffer should fail.
+	 */
+	result = isc_buffer_copyregion(b, &r);
+	ATF_CHECK_EQ(result, ISC_R_NOSPACE);
+
+	/*
+	 * Enable auto reallocation and retry.  Appending should now succeed.
+	 */
+	isc_buffer_setautorealloc(b, true);
+	result = isc_buffer_copyregion(b, &r);
+	ATF_CHECK_EQ(result, ISC_R_SUCCESS);
+
+	isc_buffer_free(&b);
+
+	isc_test_end();
+}
+
 ATF_TC(isc_buffer_printf);
 ATF_TC_HEAD(isc_buffer_printf, tc) {
 	atf_tc_set_md_var(tc, "descr", "printf() into a buffer");
@@ -278,6 +326,7 @@ ATF_TC_BODY(isc_buffer_printf, tc) {
 ATF_TP_ADD_TCS(tp) {
 	ATF_TP_ADD_TC(tp, isc_buffer_reserve);
 	ATF_TP_ADD_TC(tp, isc_buffer_dynamic);
+	ATF_TP_ADD_TC(tp, isc_buffer_copyregion);
 	ATF_TP_ADD_TC(tp, isc_buffer_printf);
 	return (atf_no_error());
 }