diff --git a/control/modelsimp.py b/control/modelsimp.py
index 4cfcf4048..8f6124481 100644
--- a/control/modelsimp.py
+++ b/control/modelsimp.py
@@ -407,7 +407,7 @@ def markov(Y, U, m=None, transpose=None):
         y[k] &= C x[k] + D u[k]
 
     given data for u and y.  The algorithm assumes that that C A^k B = 0 for
-    k > m-2 (see [1]).  Note that the problem is ill-posed if the length of
+    k > m-2 (see [1]_).  Note that the problem is ill-posed if the length of
     the input data is less than the desired number of Markov parameters (a
     warning message is generated in this case).
 
diff --git a/doc-requirements.txt b/doc-requirements.txt
index 112ca8cbe..cf1a3a76e 100644
--- a/doc-requirements.txt
+++ b/doc-requirements.txt
@@ -1,3 +1,4 @@
+sphinx>=3.4
 numpy
 scipy
 matplotlib