diff --git a/internal/disk/disk.go b/internal/disk/disk.go
index 320580d02..a088b0024 100644
--- a/internal/disk/disk.go
+++ b/internal/disk/disk.go
@@ -120,6 +120,26 @@ func (pt PartitionTable) RootPartition() *Partition {
 	return nil
 }
 
+// Returns the index of the boot partition: the partition whose filesystem has
+// /boot as a mountpoint.  If there is no explicit boot partition, the root
+// partition is returned.
+// If neither boot nor root partitions are found, returns -1.
+func (pt PartitionTable) BootPartitionIndex() int {
+	// find partition with '/boot' mountpoint and fallback to '/'
+	rootIdx := -1
+	for idx, part := range pt.Partitions {
+		if part.Filesystem == nil {
+			continue
+		}
+		if part.Filesystem.Mountpoint == "/boot" {
+			return idx
+		} else if part.Filesystem.Mountpoint == "/" {
+			rootIdx = idx
+		}
+	}
+	return rootIdx
+}
+
 // Converts Partition to osbuild.QEMUPartition that encodes the same partition.
 func (p Partition) QEMUPartition() osbuild.QEMUPartition {
 	var fs *osbuild.QEMUFilesystem
diff --git a/internal/distro/rhel85/stage_options.go b/internal/distro/rhel85/stage_options.go
index aec6819fb..39c8970b0 100644
--- a/internal/distro/rhel85/stage_options.go
+++ b/internal/distro/rhel85/stage_options.go
@@ -442,7 +442,10 @@ func copyFSTreeOptions(inputName, inputPipeline string, pt *disk.PartitionTable,
 }
 
 func grub2InstStageOptions(filename string, pt *disk.PartitionTable, platform string) *osbuild.Grub2InstStageOptions {
-	bootPartIndex := findBootPartition(pt)
+	bootPartIndex := pt.BootPartitionIndex()
+	if bootPartIndex == -1 {
+		panic("failed to find boot or root partition for grub2.inst stage")
+	}
 	core := osbuild.CoreMkImage{
 		Type:       "mkimage",
 		PartLabel:  pt.Type,
@@ -452,7 +455,7 @@ func grub2InstStageOptions(filename string, pt *disk.PartitionTable, platform st
 	prefix := osbuild.PrefixPartition{
 		Type:      "partition",
 		PartLabel: pt.Type,
-		Number:    bootPartIndex,
+		Number:    uint(bootPartIndex),
 		Path:      "/boot/grub2",
 	}
 
@@ -465,25 +468,6 @@ func grub2InstStageOptions(filename string, pt *disk.PartitionTable, platform st
 	}
 }
 
-func findBootPartition(pt *disk.PartitionTable) uint {
-	// find partition with '/boot' mountpoint and fallback to '/'
-	rootIdx := -1
-	for idx, part := range pt.Partitions {
-		if part.Filesystem == nil {
-			continue
-		}
-		if part.Filesystem.Mountpoint == "/boot" {
-			return uint(idx)
-		} else if part.Filesystem.Mountpoint == "/" {
-			rootIdx = idx
-		}
-	}
-	if rootIdx == -1 {
-		panic("failed to find boot or root partition for grub2.inst stage")
-	}
-	return uint(rootIdx)
-}
-
 func qemuStageOptions(filename, format, compat string) *osbuild.QEMUStageOptions {
 	var options osbuild.QEMUFormatOptions
 	switch format {