diff --git a/numpy_financial/_financial.py b/numpy_financial/_financial.py
index cd6bf4e..08e2ad6 100644
--- a/numpy_financial/_financial.py
+++ b/numpy_financial/_financial.py
@@ -295,8 +295,6 @@ def _nper_native(rates, pmts, pvs, fvs, whens, out):
                         )
 
 
-
-
 def nper(rate, pmt, pv, fv=0, when='end'):
     """Compute the number of periodic payments.
 
@@ -366,7 +364,27 @@ def nper(rate, pmt, pv, fv=0, when='end'):
     fv_inner = np.atleast_1d(fv)
     when_inner = np.atleast_1d(when)
 
-    # TODO: Validate ``*_inner`` array shapes
+    # TODO: I don't like repeating myself this often, refactor into a function
+    #       that checks all of the arrays at once.
+    if rate_inner.ndim != 1:
+        msg = "invalid shape for rates. Rate must be either a scalar or 1d array"
+        raise ValueError(msg)
+
+    if pmt_inner.ndim != 1:
+        msg = "invalid shape for pmt. Payments must be either a scalar or 1d array"
+        raise ValueError(msg)
+
+    if pv_inner.ndim != 1:
+        msg = "invalid shape for pv. Present value must be either a scalar or 1d array"
+        raise ValueError(msg)
+
+    if fv_inner.ndim != 1:
+        msg = "invalid shape for fv. Future value must be either a scalar or 1d array"
+        raise ValueError(msg)
+
+    if when_inner.ndim != 1:
+        msg = "invalid shape for when. When must be either a scalar or 1d array"
+        raise ValueError(msg)
 
     out_shape = _get_output_array_shape(
         rate_inner, pmt_inner, pv_inner, fv_inner, when_inner