diff --git a/CHANGES b/CHANGES
index aca743c578..b27b290107 100644
--- a/CHANGES
+++ b/CHANGES
@@ -1,3 +1,5 @@
+4218.	[bug]		Potential null pointer dereference on out of memory				if mmap is not supported. [RT #40777]
+
 4217.	[protocol]	Add support for CSYNC. [RT #40532]
 
 4216.	[cleanup]	Silence static analysis warnings. [RT #40649]
diff --git a/lib/isc/unix/file.c b/lib/isc/unix/file.c
index 4be9eced75..f0dc9fdf1c 100644
--- a/lib/isc/unix/file.c
+++ b/lib/isc/unix/file.c
@@ -674,6 +674,9 @@ isc_file_mmap(void *addr, size_t len, int prot,
 		len = end - offset;
 
 	buf = malloc(len);
+	if (buf == NULL)
+		return (NULL);
+
 	ret = read(fd, buf, len);
 	if (ret != (ssize_t) len) {
 		free(buf);
diff --git a/lib/isc/win32/file.c b/lib/isc/win32/file.c
index c6adaae20f..572dce4a5a 100644
--- a/lib/isc/win32/file.c
+++ b/lib/isc/win32/file.c
@@ -751,6 +751,9 @@ isc_file_mmap(void *addr, size_t len, int prot,
 		len = end - offset;
 
 	buf = malloc(len);
+	if (buf == NULL)
+		return (NULL);
+
 	ret = read(fd, buf, (unsigned int) len);
 	if (ret != (ssize_t) len) {
 		free(buf);