diff --git a/workflow/yaml.go b/workflow/yaml.go
index 5ed051a..735e08b 100644
--- a/workflow/yaml.go
+++ b/workflow/yaml.go
@@ -61,21 +61,31 @@ const YamlOneOfTag = "!oneof"
 
 func buildOneOfExpressions(data yaml.Node, path []string) (any, error) {
 	if data.Type() != yaml.TypeIDMap {
-		return nil, fmt.Errorf("!oneof found on non-map node at %s; expected a map with a list of options and the discriminator ", strings.Join(path, " -> "))
+		return nil, fmt.Errorf(
+			"!oneof found on non-map node at %s; expected a map with a list of options and the discriminator ",
+			strings.Join(path, " -> "))
 	}
 	discriminatorNode, found := data.MapKey(YamlDiscriminatorKey)
 	if !found {
-		return nil, fmt.Errorf("key %q not present within !oneof at %q", YamlDiscriminatorKey, strings.Join(path, " -> "))
+		return nil, fmt.Errorf("key %q not present within %s at %q",
+			YamlDiscriminatorKey, YamlOneOfTag, strings.Join(path, " -> "))
 	}
 	if discriminatorNode.Type() != yaml.TypeIDString {
-		return nil, fmt.Errorf("%q within !oneof should be a string; got %s", discriminatorNode.Type(), YamlDiscriminatorKey)
+		return nil, fmt.Errorf("%q within %s should be a string; got %s",
+			YamlDiscriminatorKey, YamlOneOfTag, discriminatorNode.Type())
+	}
+	discriminator := discriminatorNode.Value()
+	if len(discriminator) == 0 {
+		return nil, fmt.Errorf("%q within %s is empty", YamlDiscriminatorKey, YamlOneOfTag)
 	}
 	oneOfOptionsNode, found := data.MapKey(YamlOneOfKey)
 	if !found {
-		return nil, fmt.Errorf("key %q not present within !oneof at %q", YamlOneOfKey, strings.Join(path, " -> "))
+		return nil, fmt.Errorf("key %q not present within %s at %q",
+			YamlOneOfKey, YamlOneOfTag, strings.Join(path, " -> "))
 	}
 	if oneOfOptionsNode.Type() != yaml.TypeIDMap {
-		return nil, fmt.Errorf("%q within !oneof should be a map; got %s", YamlOneOfKey, discriminatorNode.Type())
+		return nil, fmt.Errorf("%q within %q should be a map; got %s",
+			YamlOneOfKey, YamlOneOfTag, discriminatorNode.Type())
 	}
 	options := map[string]any{}
 	for _, optionNodeKey := range oneOfOptionsNode.MapKeys() {
@@ -87,7 +97,6 @@ func buildOneOfExpressions(data yaml.Node, path []string) (any, error) {
 		}
 	}
 
-	discriminator := discriminatorNode.Value()
 	return &infer.OneOfExpression{
 		Discriminator: discriminator,
 		Options:       options,
diff --git a/workflow/yaml_test.go b/workflow/yaml_test.go
new file mode 100644
index 0000000..8389f2a
--- /dev/null
+++ b/workflow/yaml_test.go
@@ -0,0 +1,105 @@
+package workflow //nolint:testpackage // Tests internal functions for unit testing.
+
+import (
+	"go.arcalot.io/assert"
+	"go.arcalot.io/lang"
+	"go.flow.arcalot.io/engine/internal/infer"
+	"go.flow.arcalot.io/engine/internal/yaml"
+	"go.flow.arcalot.io/expressions"
+	"testing"
+)
+
+func TestBuildOneOfExpression_Simple(t *testing.T) {
+	yamlInput := []byte(`
+!oneof
+  discriminator: d
+  one_of:
+    a: !expr some_expr
+    b: !expr some_other_expr
+`)
+	input := assert.NoErrorR[yaml.Node](t)(yaml.New().Parse(yamlInput))
+	result, err := buildOneOfExpressions(input, make([]string, 0))
+	assert.NoError(t, err)
+	assert.InstanceOf[*infer.OneOfExpression](t, result)
+	oneofResult := result.(*infer.OneOfExpression)
+	assert.Equals(t, oneofResult.Discriminator, "d")
+	assert.Equals(t, oneofResult.Options, map[string]any{
+		"a": lang.Must2(expressions.New("some_expr")),
+		"b": lang.Must2(expressions.New("some_other_expr")),
+	})
+}
+
+func TestBuildOneOfExpression_InputValidation(t *testing.T) {
+	// Not a map
+	yamlInput := []byte(`
+!oneof "thisisastring"`)
+	input := assert.NoErrorR[yaml.Node](t)(yaml.New().Parse(yamlInput))
+	_, err := buildOneOfExpressions(input, make([]string, 0))
+	assert.Error(t, err)
+	assert.Contains(t, err.Error(), "expected a map")
+
+	// Wrong discriminator key
+	yamlInput = []byte(`
+!oneof
+  wrong_key: ""
+  one_of: {}`)
+	input = assert.NoErrorR[yaml.Node](t)(yaml.New().Parse(yamlInput))
+	_, err = buildOneOfExpressions(input, make([]string, 0))
+	assert.Error(t, err)
+	assert.Contains(t, err.Error(), "key \""+YamlDiscriminatorKey+"\" not present")
+
+	// Discriminator not a string
+	yamlInput = []byte(`
+!oneof
+  discriminator: {}
+  one_of: {}`)
+	input = assert.NoErrorR[yaml.Node](t)(yaml.New().Parse(yamlInput))
+	_, err = buildOneOfExpressions(input, make([]string, 0))
+	assert.Error(t, err)
+	assert.Contains(t, err.Error(), "should be a string")
+
+	// Empty discriminator
+	yamlInput = []byte(`
+!oneof
+  discriminator: ""
+  one_of: {}`)
+	input = assert.NoErrorR[yaml.Node](t)(yaml.New().Parse(yamlInput))
+	_, err = buildOneOfExpressions(input, make([]string, 0))
+	assert.Error(t, err)
+	assert.Contains(t, err.Error(), "is empty")
+
+	// Missing or wrong oneof key
+	yamlInput = []byte(`
+!oneof
+  discriminator: "valid"
+  one_of_wrong: {}`)
+	input = assert.NoErrorR[yaml.Node](t)(yaml.New().Parse(yamlInput))
+	_, err = buildOneOfExpressions(input, make([]string, 0))
+	assert.Error(t, err)
+	assert.Contains(t, err.Error(), "key \"one_of\" not present")
+
+	// Wrong type for oneof options node
+	yamlInput = []byte(`
+!oneof
+  discriminator: "valid"
+  one_of: wrong`)
+	input = assert.NoErrorR[yaml.Node](t)(yaml.New().Parse(yamlInput))
+	_, err = buildOneOfExpressions(input, make([]string, 0))
+	assert.Error(t, err)
+	assert.Contains(t, err.Error(), "should be a map")
+
+	// Non-object type as option in one_of section.
+	yamlInput = []byte(`
+!oneof
+  discriminator: "valid"
+  one_of:
+    a: test`)
+	input = assert.NoErrorR[yaml.Node](t)(yaml.New().Parse(yamlInput))
+	oneofResult, err := buildOneOfExpressions(input, make([]string, 0))
+	assert.NoError(t, err)
+	assert.InstanceOf[*infer.OneOfExpression](t, oneofResult)
+	_, err = oneofResult.(*infer.OneOfExpression).Type(nil, nil, nil)
+	assert.Error(t, err)
+	assert.Contains(t, err.Error(), "not an object")
+
+}