diff --git a/usr.bin/mkimg/image.c b/usr.bin/mkimg/image.c
index 2527f289184..39ae4123080 100644
--- a/usr.bin/mkimg/image.c
+++ b/usr.bin/mkimg/image.c
@@ -456,8 +456,7 @@ image_copyin_mapped(lba_t blk, int fd, uint64_t *sizep)
 			 * I don't know what this means or whether it
 			 * can happen at all...
 			 */
-			error = EDOOFUS;
-			break;
+			assert(0);
 		}
 	}
 	if (error)
@@ -602,7 +601,7 @@ image_copyout_region(int fd, lba_t blk, lba_t size)
 			error = image_copyout_memory(fd, sz, ch->ch_u.mem.ptr);
 			break;
 		default:
-			return (EDOOFUS);
+			assert(0);
 		}
 		size -= sz;
 		blk += sz / secsz;
diff --git a/usr.bin/mkimg/qcow.c b/usr.bin/mkimg/qcow.c
index a89761b8010..64c437a9202 100644
--- a/usr.bin/mkimg/qcow.c
+++ b/usr.bin/mkimg/qcow.c
@@ -30,6 +30,7 @@ __FBSDID("$FreeBSD$");
 #include <sys/types.h>
 #include <sys/endian.h>
 #include <sys/errno.h>
+#include <assert.h>
 #include <stdint.h>
 #include <stdio.h>
 #include <stdlib.h>
@@ -102,7 +103,7 @@ qcow_resize(lba_t imgsz, u_int version)
 		clstr_log2sz = QCOW2_CLSTR_LOG2SZ;
 		break;
 	default:
-		return (EDOOFUS);
+		assert(0);
 	}
 
 	imagesz = round_clstr(imgsz * secsz);
@@ -143,8 +144,7 @@ qcow_write(int fd, u_int version)
 	u_int clstrsz, l1idx, l2idx;
 	int error;
 
-	if (clstr_log2sz == 0)
-		return (EDOOFUS);
+	assert(clstr_log2sz != 0);
 
 	clstrsz = 1U << clstr_log2sz;
 	blk_clstrsz = clstrsz / secsz;
@@ -203,7 +203,7 @@ qcow_write(int fd, u_int version)
 		be32enc(&hdr->u.v2.refcnt_clstrs, refcnt_clstrs);
 		break;
 	default:
-		return (EDOOFUS);
+		assert(0);
 	}
 
 	if (sparse_write(fd, hdr, clstrsz) < 0) {