diff --git a/assemblers/org.osbuild.qemu b/assemblers/org.osbuild.qemu
index 184f521c..0ee8d97f 100755
--- a/assemblers/org.osbuild.qemu
+++ b/assemblers/org.osbuild.qemu
@@ -82,7 +82,7 @@ def mkfs_for_type(device, uuid, fs_type):
     elif fs_type == "xfs":
         maker = mkfs_xfs
     else:
-        raise ValueError("Unknown filesystem type")
+        raise ValueError(f"Unknown filesystem type '{fs_type}'")
     maker(device, uuid)
 
 
@@ -112,10 +112,23 @@ def create_partition_table(image, options):
     return partitions
 
 
-def install_grub2(image, fs_module, partition_offset):
+def install_grub2(image, partitions):
     """Install grub2 to image"""
     grub2_core = "/var/tmp/grub2-core.img"
 
+    root_fs_type = "unknown"
+    for p in partitions:
+        if p["filesystem"]["mountpoint"] == "/":
+            root_fs_type = p["filesystem"]["type"]
+            break
+
+    if root_fs_type == "ext4":
+        fs_module = "ext2"
+    elif root_fs_type == "xfs":
+        fs_module = "xfs"
+    else:
+        raise ValueError(f"unknown root filesystem type: '{root_fs_type}'")
+
     # Create the level-2 bootloader
     # The purpose of this is to find the grub modules and configuration
     # to be able to start the level-3 bootloader. It contains the modules
@@ -130,6 +143,7 @@ def install_grub2(image, fs_module, partition_offset):
                     "part_msdos", fs_module, "biosdisk"],
                    check=True)
 
+    partition_offset = partitions[0]["start"]
     assert os.path.getsize(grub2_core) < partition_offset - 512
 
     with open(image, "rb+") as image_f:
@@ -153,7 +167,6 @@ def main(tree, output_dir, options, loop_client):
     fmt = options["format"]
     filename = options["filename"]
     size = options["size"]
-    root_fs_type = options.get("root_fs_type", "ext4")
 
     # sfdisk works on sectors of 512 bytes and ignores excess space - be explicit about this
     if size % 512 != 0:
@@ -162,15 +175,6 @@ def main(tree, output_dir, options, loop_client):
     if fmt not in ["raw", "raw.xz", "qcow2", "vdi", "vmdk", "vpc"]:
         raise ValueError("`format` must be one of raw, qcow, vdi, vmdk, vpc")
 
-    if root_fs_type == "ext4":
-        mkfs = mkfs_ext4
-        grub2_fs_module = "ext2"
-    elif root_fs_type == "xfs":
-        mkfs = mkfs_xfs
-        grub2_fs_module = "xfs"
-    else:
-        raise ValueError("`root_fs_type` must be either ext4 or xfs")
-
     image = "/var/tmp/osbuild-image.raw"
 
     # Create an empty image file
@@ -178,10 +182,9 @@ def main(tree, output_dir, options, loop_client):
 
     # The partition table
     partitions = create_partition_table(image, options)
-    partition_offset = partitions[0]["start"]
 
     # Create the level-2 bootloader
-    install_grub2(image, grub2_fs_module, partition_offset)
+    install_grub2(image, partitions)
 
     # Now assemble the filesystem hierarchy and copy the tree into the image
     with contextlib.ExitStack() as cm: