diff --git a/src/4_sommerfeld_model_solutions.md b/src/4_sommerfeld_model_solutions.md
index b661c05b19cd7e07e8bf81d11fe017813d97005e..3772ce8bfb16ae925fd2d584a77f9a486125e25e 100644
--- a/src/4_sommerfeld_model_solutions.md
+++ b/src/4_sommerfeld_model_solutions.md
@@ -130,6 +130,8 @@ CV = (dEplus - dEmin) / (2*dT);
 1.
 
 ```python
+import numpy as np
+import matplotlib.pyplot as plt
 x = np.linspace(-1, 1, 100)
 fig, ax = plt.subplots(figsize=(7, 5))
 ax.plot(x, x, 'b')