diff --git a/src/nvda_voicemeeter/errors.py b/src/nvda_voicemeeter/errors.py
new file mode 100644
index 0000000..a0b7ccc
--- /dev/null
+++ b/src/nvda_voicemeeter/errors.py
@@ -0,0 +1,11 @@
+class NVDAVMError(Exception):
+    """Base NVDAVM error class"""
+
+
+class NVDAVMCAPIError(NVDAVMError):
+    """Exception raised when the NVDA C-API returns an error code"""
+
+    def __init__(self, fn_name, code):
+        self.fn_name = fn_name
+        self.code = code
+        super().__init__(f"{self.fn_name} returned {self.code}")
diff --git a/src/nvda_voicemeeter/nvda.py b/src/nvda_voicemeeter/nvda.py
index 31753ea..bed8246 100644
--- a/src/nvda_voicemeeter/nvda.py
+++ b/src/nvda_voicemeeter/nvda.py
@@ -1,4 +1,5 @@
 from .cdll import libc
+from .errors import NVDAVMCAPIError
 
 
 class CBindings:
@@ -7,17 +8,20 @@ class CBindings:
     bind_cancel_speech = libc.nvdaController_cancelSpeech
     bind_braille_message = libc.nvdaController_brailleMessage
 
-    def call(self, fn, *args, ok=(0,)):
+    def call(self, fn, *args, ok=(0,), ok_exp=None):
         retval = fn(*args)
-        if retval not in ok:
-            raise RuntimeError(f"{fn.__name__} returned {retval}")
+        if ok_exp is None:
+            if retval not in ok:
+                raise NVDAVMCAPIError(fn.__name__, retval)
+        elif not ok_exp(retval) and retval not in ok:
+            raise NVDAVMCAPIError(fn.__name__, retval)
         return retval
 
 
 class Nvda(CBindings):
     @property
     def is_running(self):
-        return self.call(self.bind_test_if_running, ok=(0, 1)) == 0
+        return self.call(self.bind_test_if_running, ok_exp=lambda x: x >= 0) == 0
 
     def speak(self, text):
         self.call(self.bind_speak_text, text)