diff --git a/Bootloaders/makefile b/Bootloaders/makefile
index 1a4d3457e0..003069b9ae 100644
--- a/Bootloaders/makefile
+++ b/Bootloaders/makefile
@@ -13,7 +13,7 @@
 # custom LUFA library build options are reflected in the compiled
 # code.
 
-PROJECT_DIRECTORIES = $(shell ls -d */)
+PROJECT_DIRECTORIES := $(shell ls -d */)
 
 # This makefile is potentially infinitely recursive if something really bad
 # happens when determining the set of project directories - hard-abort if
diff --git a/Demos/Device/ClassDriver/makefile b/Demos/Device/ClassDriver/makefile
index 797a483fa3..776288f298 100644
--- a/Demos/Device/ClassDriver/makefile
+++ b/Demos/Device/ClassDriver/makefile
@@ -13,7 +13,7 @@
 # custom LUFA library build options are reflected in the compiled
 # code.
 
-PROJECT_DIRECTORIES = $(shell ls -d */)
+PROJECT_DIRECTORIES := $(shell ls -d */)
 
 # This makefile is potentially infinitely recursive if something really bad
 # happens when determining the set of project directories - hard-abort if
diff --git a/Demos/Device/LowLevel/makefile b/Demos/Device/LowLevel/makefile
index 797a483fa3..776288f298 100644
--- a/Demos/Device/LowLevel/makefile
+++ b/Demos/Device/LowLevel/makefile
@@ -13,7 +13,7 @@
 # custom LUFA library build options are reflected in the compiled
 # code.
 
-PROJECT_DIRECTORIES = $(shell ls -d */)
+PROJECT_DIRECTORIES := $(shell ls -d */)
 
 # This makefile is potentially infinitely recursive if something really bad
 # happens when determining the set of project directories - hard-abort if
diff --git a/Demos/DualRole/ClassDriver/makefile b/Demos/DualRole/ClassDriver/makefile
index b9538fe3b8..6d16493ee9 100644
--- a/Demos/DualRole/ClassDriver/makefile
+++ b/Demos/DualRole/ClassDriver/makefile
@@ -13,7 +13,7 @@
 # custom LUFA library build options are reflected in the compiled
 # code.
 
-PROJECT_DIRECTORIES = $(shell ls -d */)
+PROJECT_DIRECTORIES := $(shell ls -d */)
 
 # This makefile is potentially infinitely recursive if something really bad
 # happens when determining the set of project directories - hard-abort if
diff --git a/Demos/Host/ClassDriver/makefile b/Demos/Host/ClassDriver/makefile
index d057a1c698..e33bf963bd 100644
--- a/Demos/Host/ClassDriver/makefile
+++ b/Demos/Host/ClassDriver/makefile
@@ -13,7 +13,7 @@
 # custom LUFA library build options are reflected in the compiled
 # code.
 
-PROJECT_DIRECTORIES = $(shell ls -d */)
+PROJECT_DIRECTORIES := $(shell ls -d */)
 
 # This makefile is potentially infinitely recursive if something really bad
 # happens when determining the set of project directories - hard-abort if
diff --git a/Demos/Host/LowLevel/makefile b/Demos/Host/LowLevel/makefile
index d057a1c698..e33bf963bd 100644
--- a/Demos/Host/LowLevel/makefile
+++ b/Demos/Host/LowLevel/makefile
@@ -13,7 +13,7 @@
 # custom LUFA library build options are reflected in the compiled
 # code.
 
-PROJECT_DIRECTORIES = $(shell ls -d */)
+PROJECT_DIRECTORIES := $(shell ls -d */)
 
 # This makefile is potentially infinitely recursive if something really bad
 # happens when determining the set of project directories - hard-abort if
diff --git a/Projects/makefile b/Projects/makefile
index f4b411145a..dd0ea2e8f5 100644
--- a/Projects/makefile
+++ b/Projects/makefile
@@ -13,7 +13,7 @@
 # custom LUFA library build options are reflected in the compiled
 # code.
 
-PROJECT_DIRECTORIES = $(shell ls -d */)
+PROJECT_DIRECTORIES := $(shell ls -d */)
 
 # This makefile is potentially infinitely recursive if something really bad
 # happens when determining the set of project directories - hard-abort if