diff --git a/rules/pulpos/default_rules.mk b/rules/pulpos/default_rules.mk
index 873d1e6..210a7f7 100644
--- a/rules/pulpos/default_rules.mk
+++ b/rules/pulpos/default_rules.mk
@@ -212,15 +212,31 @@ endif
 ifeq '$(platform)' 'rtl'
 
 $(TARGET_BUILD_DIR)/modelsim.ini:
+ifndef VSIM_PATH
+	$(error "VSIM_PATH is undefined. Either call \
+	'source $$YOUR_HW_DIR/setup/vsim.sh' or set it manually.")
+endif
 	ln -s $(VSIM_PATH)/modelsim.ini $@
 
 $(TARGET_BUILD_DIR)/boot:
+ifndef VSIM_PATH
+	$(error "VSIM_PATH is undefined. Either call \
+	'source $$YOUR_HW_DIR/setup/vsim.sh' or set it manually.")
+endif
 	ln -s $(VSIM_PATH)/boot $@
 
 $(TARGET_BUILD_DIR)/tcl_files:
+ifndef VSIM_PATH
+	$(error "VSIM_PATH is undefined. Either call \
+	'source $$YOUR_HW_DIR/setup/vsim.sh' or set it manually.")
+endif
 	ln -s $(VSIM_PATH)/tcl_files $@
 
 $(TARGET_BUILD_DIR)/waves:
+ifndef VSIM_PATH
+	$(error "VSIM_PATH is undefined. Either call \
+	'source $$YOUR_HW_DIR/setup/vsim.sh' or set it manually.")
+endif
 	ln -s $(VSIM_PATH)/waves $@
 
 $(TARGET_BUILD_DIR)/stdout:
@@ -232,7 +248,10 @@ $(TARGET_BUILD_DIR)/fs:
 run: $(TARGET_BUILD_DIR)/modelsim.ini  $(TARGET_BUILD_DIR)/boot $(TARGET_BUILD_DIR)/tcl_files \
 	$(TARGET_BUILD_DIR)/stdout $(TARGET_BUILD_DIR)/fs
 	$(PULPRT_HOME)/bin/stim_utils.py --binary=$(TARGETS) --vectors=$(TARGET_BUILD_DIR)/vectors/stim.txt
-
+ifndef VSIM_PATH
+	$(error "VSIM_PATH is undefined. Either call \
+	'source $$YOUR_HW_DIR/setup/vsim.sh' or set it manually.")
+endif
 ifdef gui
 	cd $(TARGET_BUILD_DIR) && export VSIM_RUNNER_FLAGS="+ENTRY_POINT=0x1c008080 -gLOAD_L2=JTAG -dpicpppath /usr/bin/g++ -permit_unmatched_virtual_intf -gBAUDRATE=115200" && export VOPT_ACC_ENA="YES" && vsim -64 -do 'source $(VSIM_PATH)/tcl_files/config/run_and_exit.tcl' -do 'source $(VSIM_PATH)/tcl_files/run.tcl; '
 else