diff --git a/src/fitness/supervised_learning/supervised_learning.py b/src/fitness/supervised_learning/supervised_learning.py
index 15a5eed9..81e54dc3 100644
--- a/src/fitness/supervised_learning/supervised_learning.py
+++ b/src/fitness/supervised_learning/supervised_learning.py
@@ -89,8 +89,9 @@ def evaluate(self, ind, **kwargs):
                 # phen will refer to x (ie test_in), and possibly to c
                 yhat = eval(phen)
                 assert np.isrealobj(yhat)
-                if y.shape != yhat.shape:
-                    raise ValueError(shape_mismatch_txt)
+                if not np.isscalar(yhat):
+                    if y.shape != yhat.shape:
+                        raise ValueError(shape_mismatch_txt)
 
                 # let's always call the error function with the
                 # true values first, the estimate second
@@ -100,8 +101,9 @@ def evaluate(self, ind, **kwargs):
             # phenotype won't refer to C
             yhat = eval(ind.phenotype)
             assert np.isrealobj(yhat)
-            if y.shape != yhat.shape:
-                raise ValueError(shape_mismatch_txt)
+            if not np.isscalar(yhat):
+                if y.shape != yhat.shape:
+                    raise ValueError(shape_mismatch_txt)
 
             # let's always call the error function with the true
             # values first, the estimate second
diff --git a/src/utilities/fitness/optimize_constants.py b/src/utilities/fitness/optimize_constants.py
index 65f64432..051461cc 100644
--- a/src/utilities/fitness/optimize_constants.py
+++ b/src/utilities/fitness/optimize_constants.py
@@ -40,8 +40,9 @@ def optimize_constants(x, y, ind):
         # ind doesn't refer to c: no need to optimize
         c = []
         yhat = f(x, c)
-        if y.shape != yhat.shape:
-            raise ValueError(shape_mismatch_txt)
+        if not np.isscalar(yhat):
+            if y.shape != yhat.shape:
+                raise ValueError(shape_mismatch_txt)
         fitness = loss(y, yhat)
         ind.opt_consts = c
         return fitness