diff --git a/README.md b/README.md
index a3933a7..a5d324b 100644
--- a/README.md
+++ b/README.md
@@ -49,8 +49,8 @@ to PR here.
 * ```python
   npx.unique_rows(a, return_inverse=False, return_counts=False)
   ```
-  Returns the unique rows of the array `a`. The numpy alternative `np.unique(a, axis=0)`
-  [is slow](https://github.com/numpy/numpy/issues/11136).
+  Returns the unique rows of the integer array `a`. The numpy alternative `np.unique(a,
+  axis=0)` [is slow](https://github.com/numpy/numpy/issues/11136).
 
 ### License
 npx is published under the [MIT license](https://en.wikipedia.org/wiki/MIT_License).
diff --git a/npx/main.py b/npx/main.py
index cf11838..0fbfe99 100644
--- a/npx/main.py
+++ b/npx/main.py
@@ -84,6 +84,9 @@ def unique_rows(a, return_inverse=False, return_counts=False):
     # The numpy alternative `np.unique(a, axis=0)` is slow; cf.
     # <https://github.com/numpy/numpy/issues/11136>.
     a = np.asarray(a)
+    if not np.issubdtype(a.dtype, np.integer):
+        raise ValueError(f"Input array must be integer type, got {a.dtype}.")
+
     b = np.ascontiguousarray(a).view(np.dtype((np.void, a.dtype.itemsize * a.shape[1])))
     out = np.unique(b, return_inverse=return_inverse, return_counts=return_counts)
     # out[0] are the sorted, unique rows
diff --git a/test/test_npx.py b/test/test_npx.py
index d7a3115..5611e57 100644
--- a/test/test_npx.py
+++ b/test/test_npx.py
@@ -1,4 +1,5 @@
 import numpy as np
+import pytest
 
 import npx
 
@@ -60,3 +61,7 @@ def test_unique_rows():
 
     a_unique = npx.unique_rows(a)
     assert np.all(a_unique == [[1, 2], [1, 4]])
+
+    a = [1.1, 2.2]
+    with pytest.raises(ValueError):
+        a_unique = npx.unique_rows(a)