diff --git a/bilby/core/prior.py b/bilby/core/prior.py
index be34de2031e1f4ed0f9a37a257037b64dde89011..15ece0185011f25ca0ecf52dfd498797b60bd7ee 100644
--- a/bilby/core/prior.py
+++ b/bilby/core/prior.py
@@ -857,9 +857,47 @@ class Prior(object):
 
     @classmethod
     def _parse_argument_string(cls, val):
-        if re.sub(r'\'.*\'', '', val) in ['r', 'u']:
-            # If the val is a latex label like "r"\log(x)"' then ignore it
-            pass
+        """
+        Parse a string into the appropriate type for prior reading.
+
+        Four tests are applied in the following order:
+
+        - If the string is 'None':
+            `None` is returned.
+        - Else If the string is a raw string, e.g., r'foo':
+            A stripped version of the string is returned, e.g., foo.
+        - Else If the string contains ', e.g., 'foo':
+            A stripped version of the string is returned, e.g., foo.
+        - Else If the string contains an open parenthesis, (:
+            The string is interpreted as a call to instantiate another prior
+            class, Bilby will attempt to recursively construct that prior,
+            e.g., Uniform(minimum=0, maximum=1), my.custom.PriorClass(**kwargs).
+        - Else:
+            Try to evaluate the string using `eval`. Only built-in functions
+            and numpy methods can be used, e.g., np.pi / 2, 1.57.
+
+
+        Parameters
+        ----------
+        val: str
+            The string version of the agument
+
+        Returns
+        -------
+        val: object
+            The parsed version of the argument.
+
+        Raises
+        ------
+        TypeError:
+            If val cannot be parsed as described above.
+        """
+        if val == 'None':
+            val = None
+        elif re.sub(r'\'.*\'', '', val) in ['r', 'u']:
+            val = val[2:-1]
+        elif "'" in val:
+            val = val.strip("'")
         elif '(' in val:
             other_cls = val.split('(')[0]
             vals = '('.join(val.split('(')[1:])[:-1]
@@ -870,15 +908,14 @@ class Prior(object):
                 module = __name__
             other_cls = getattr(import_module(module), other_cls)
             val = other_cls.from_repr(vals)
-        elif "'" in val:
-            val = val.strip("'")
-        elif val == 'None':
-            val = None
         else:
             try:
                 val = eval(val, dict(), dict(np=np))
             except NameError:
-                raise TypeError()
+                raise TypeError(
+                    "Cannot evaluate prior, "
+                    "failed to parse argument {}".format(val)
+                )
         return val
 
 
diff --git a/test/prior_files/prior_with_parentheses.prior b/test/prior_files/prior_with_parentheses.prior
index 2430db3cac0f749a81c6a3dc27a0ca0e5de85fdb..5296497f70f5b51ac0fde4c6633a8a82e9d5ca44 100644
--- a/test/prior_files/prior_with_parentheses.prior
+++ b/test/prior_files/prior_with_parentheses.prior
@@ -1 +1,2 @@
 logA = Uniform(name='logA', minimum=10, maximum=20, latex_label=r'$\log(A_{0})$')
+logB = Uniform(name='logB', minimum=10, maximum=20, latex_label='$\log(B_{0})$')