diff --git a/man/kernel-install.xml b/man/kernel-install.xml
index 88208250e8..f278c2f557 100644
--- a/man/kernel-install.xml
+++ b/man/kernel-install.xml
@@ -161,8 +161,12 @@
 
   <refsect1>
     <title>Environment variables</title>
+
     <para>If <option>--verbose</option> is used, <varname>$KERNEL_INSTALL_VERBOSE=1</varname> will be set for
     the plugins. They may output additional logs in this case.</para>
+
+    <para>If <varname>MACHINE_ID=</varname> is set and not empty, it will be used as <replaceable>MACHINE-ID</replaceable>,
+    overriding any automatic detection attempts.  The value must be a valid machine ID (32 hexadecimal characters).</para>
   </refsect1>
 
   <refsect1>
@@ -214,9 +218,9 @@
         </term>
           <listitem>
             <para>The content of this file specifies the machine identification
-            <replaceable>MACHINE-ID</replaceable>.  If <filename>$BOOT/Default</filename> exists,
-            or <filename>/etc/machine-id</filename> doesn't, <command>kernel-install</command>
-            will use the literal <literal>Default</literal> as the machine ID instead.</para>
+            <replaceable>MACHINE-ID</replaceable>. If <filename>/etc/machine-id</filename>
+            cannot be read or is temporary (backed by a file on <constant>tmpfs</constant>),
+            <command>kernel-install</command> will use <literal>Default</literal> instead.</para>
           </listitem>
       </varlistentry>
       <varlistentry>
diff --git a/src/kernel-install/kernel-install b/src/kernel-install/kernel-install
index c9a80b2e3d..89f0074417 100755
--- a/src/kernel-install/kernel-install
+++ b/src/kernel-install/kernel-install
@@ -85,20 +85,14 @@ fi
 KERNEL_VERSION="$1"
 KERNEL_IMAGE="$2"
 
-# Reuse directory created without a machine ID present if it exists.
-if [[ -d /efi/Default ]] || [[ -d /boot/Default ]] || [[ -d /boot/efi/Default ]]; then
-    MACHINE_ID="Default"
-elif [[ -f /etc/machine-id ]]; then
-    read MACHINE_ID < /etc/machine-id
-else
-    MACHINE_ID="Default"
-fi
-
 if [[ ! $COMMAND ]] || [[ ! $KERNEL_VERSION ]]; then
     echo "Not enough arguments" >&2
     exit 1
 fi
 
+[ -z "$MACHINE_ID" ] && [ -f /etc/machine-id ] && [ "$(stat -fc %T /etc/machine-id)" != "tmpfs" ] && read -r MACHINE_ID < /etc/machine-id
+[ -z "$MACHINE_ID" ] && MACHINE_ID="Default"
+
 if [[ -d /efi/loader/entries ]] || [[ -d /efi/$MACHINE_ID ]]; then
     ENTRY_DIR_ABS="/efi/$MACHINE_ID/$KERNEL_VERSION"
 elif [[ -d /boot/loader/entries ]] || [[ -d /boot/$MACHINE_ID ]]; then
@@ -113,7 +107,7 @@ else
     ENTRY_DIR_ABS="/boot/$MACHINE_ID/$KERNEL_VERSION"
 fi
 
-export KERNEL_INSTALL_MACHINE_ID=$MACHINE_ID
+export KERNEL_INSTALL_MACHINE_ID="$MACHINE_ID"
 
 ret=0