diff --git a/pyzx/circuit/qasmparser.py b/pyzx/circuit/qasmparser.py
index e54ae4eb..15b1b8c8 100644
--- a/pyzx/circuit/qasmparser.py
+++ b/pyzx/circuit/qasmparser.py
@@ -229,6 +229,7 @@ def parse_phase_arg(self, val):
                 else:
                     val = val.strip()
                     if not val: phase = 1
+                    elif val == '-': phase = -1
                     else: phase = float(val)
             except: raise TypeError("Invalid specification {}".format(val))
         phase = Fraction(phase).limit_denominator(100000000)