diff --git a/sys/dev/agp/agp.c b/sys/dev/agp/agp.c
index 7bf49b29c3c..e66445995d7 100644
--- a/sys/dev/agp/agp.c
+++ b/sys/dev/agp/agp.c
@@ -175,6 +175,11 @@ agp_alloc_gatt(device_t dev)
 			      "allocating GATT for aperture of size %dM\n",
 			      apsize / (1024*1024));
 
+	if (entries == 0) {
+		device_printf(dev, "bad aperture size\n");
+		return NULL;
+	}
+
 	gatt = malloc(sizeof(struct agp_gatt), M_AGP, M_NOWAIT);
 	if (!gatt)
 		return 0;
diff --git a/sys/dev/agp/agp_ali.c b/sys/dev/agp/agp_ali.c
index b9b2c1326f5..fc99574de70 100644
--- a/sys/dev/agp/agp_ali.c
+++ b/sys/dev/agp/agp_ali.c
@@ -102,6 +102,10 @@ agp_ali_attach(device_t dev)
 		return error;
 
 	sc->initial_aperture = AGP_GET_APERTURE(dev);
+	if (sc->initial_aperture == 0) {
+		device_printf(dev, "bad initial aperture size, disabling\n");
+		return ENXIO;
+	}
 
 	for (;;) {
 		gatt = agp_alloc_gatt(dev);
diff --git a/sys/pci/agp.c b/sys/pci/agp.c
index 7bf49b29c3c..e66445995d7 100644
--- a/sys/pci/agp.c
+++ b/sys/pci/agp.c
@@ -175,6 +175,11 @@ agp_alloc_gatt(device_t dev)
 			      "allocating GATT for aperture of size %dM\n",
 			      apsize / (1024*1024));
 
+	if (entries == 0) {
+		device_printf(dev, "bad aperture size\n");
+		return NULL;
+	}
+
 	gatt = malloc(sizeof(struct agp_gatt), M_AGP, M_NOWAIT);
 	if (!gatt)
 		return 0;
diff --git a/sys/pci/agp_ali.c b/sys/pci/agp_ali.c
index b9b2c1326f5..fc99574de70 100644
--- a/sys/pci/agp_ali.c
+++ b/sys/pci/agp_ali.c
@@ -102,6 +102,10 @@ agp_ali_attach(device_t dev)
 		return error;
 
 	sc->initial_aperture = AGP_GET_APERTURE(dev);
+	if (sc->initial_aperture == 0) {
+		device_printf(dev, "bad initial aperture size, disabling\n");
+		return ENXIO;
+	}
 
 	for (;;) {
 		gatt = agp_alloc_gatt(dev);