diff --git a/repos/libports/lib/mk/spec/arm/libc-gen.mk b/repos/libports/lib/mk/spec/arm/libc-gen.mk
index 780beae1f8..41b7ca6e27 100644
--- a/repos/libports/lib/mk/spec/arm/libc-gen.mk
+++ b/repos/libports/lib/mk/spec/arm/libc-gen.mk
@@ -9,7 +9,14 @@ FILTER_OUT_C += _set_tp.c fabs.c frexp.c modf.c
 SRC_S  += $(filter-out $(FILTER_OUT_S),$(notdir $(wildcard $(LIBC_GEN_ARM_DIR)/*.S)))
 SRC_C  += $(filter-out $(FILTER_OUT_C),$(notdir $(wildcard $(LIBC_GEN_ARM_DIR)/*.c)))
 
-# fix missing include prefix for 'ucontext.h', should be 'sys/ucontext.h'
-CC_OPT_makecontext = -I$(call select_from_ports,libc)/include/libc/sys
+#
+# Fix missing include prefix for 'ucontext.h', should be 'sys/ucontext.h'
+#
+# The first path is in effect when using the regular build system. The second
+# path is in effect when building the libc from a source archive (where the
+# ucontext.h header is taken from the libc API archive).
+#
+CC_OPT_makecontext = -I$(call select_from_ports,libc)/include/libc/sys \
+                     $(addprefix -I,$(call select_from_repositories,/include/libc/sys))
 
 vpath % $(LIBC_GEN_ARM_DIR)