diff --git a/cpukit/score/cpu/arm/Makefile.am b/cpukit/score/cpu/arm/Makefile.am
index f02efb599a..abf5e9dcdc 100644
--- a/cpukit/score/cpu/arm/Makefile.am
+++ b/cpukit/score/cpu/arm/Makefile.am
@@ -10,10 +10,9 @@ include_rtems_score_HEADERS = rtems/score/cpu.h rtems/score/cpu_asm.h \
     rtems/score/arm.h rtems/score/types.h
 
 noinst_LIBRARIES = libscorecpu.a
-libscorecpu_a_SOURCES = cpu.c cpu_asm.S
 libscorecpu_a_CPPFLAGS = $(AM_CPPFLAGS)
-SUBDIRS = thumb
-libscorecpu_a_LIBADD = ./thumb/libthumbisr_a-thumb_isr.o
+libscorecpu_a_SOURCES = cpu.c cpu_asm.S
+libscorecpu_a_SOURCES += thumb/thumb_isr.c
 
 include $(srcdir)/preinstall.am
 include $(top_srcdir)/automake/local.am