diff --git a/secop_psi/softcal.py b/secop_psi/softcal.py
index e7baf00..a56c865 100644
--- a/secop_psi/softcal.py
+++ b/secop_psi/softcal.py
@@ -74,13 +74,18 @@ class Parser340(StdParser):
     def parse(self, line):
         """scan header for data format"""
         if self.header:
-            if line.startswith("Data Format"):
-                dataformat = line.split(":")[1].strip()[0]
-                if dataformat == '4':
-                    self.logx, self.logy = True, False  # logOhm
-                elif dataformat == '5':
-                    self.logx, self.logy = True, True  # logOhm, logK
-            elif line.startswith("No."):
+            key, _, value = line.partition(':')
+            if value:  # this is a header line, as it contains ':'
+                value = value.split()[0]
+                key = ''.join(key.split()).lower()
+                if key == 'dataformat':
+                    if value == '4':
+                        self.logx, self.logy = True, False  # logOhm
+                    elif value == '5':
+                        self.logx, self.logy = True, True  # logOhm, logK
+                    elif value not in ('1', '2', '3'):
+                        raise ValueError('invalid Data Format')
+            elif line.contains('No.'):
                 self.header = False
             return
         super().parse(line)
@@ -140,7 +145,10 @@ class CalCurve:
                 parser.parse(line)
         self.convert_x = nplog if parser.logx else linear
         self.convert_y = npexp if parser.logy else linear
-        self.spline = splrep(np.asarray(parser.xdata), np.asarray(parser.ydata), s=0)
+        try:
+            self.spline = splrep(np.asarray(parser.xdata), np.asarray(parser.ydata), s=0)
+        except:
+            raise ValueError('invalid curve, may be not monotonic?')
 
     def __call__(self, value):
         """convert value