                         greaterThanOrEqualToSystemSpacingAfter: secondAnchor,
                         multiplier: multiplier
                     )
+                @unknown default:
+                    assertionFailure("Unknown relation '\(relation)', falling back to `.equal`.", file: file, line: line)
+                    return firstAnchor.constraint(
+                        equalToSystemSpacingAfter: secondAnchor,
+                        multiplier: multiplier
+                    )
                 }
             })
     }

                        multiplier: multiplier,
                        constant: constant
                    )
                }
            })
    }

                        multiplier: multiplier,
                        constant: constant
                    )
                @unknown default:
                    assertionFailure("Unknown relation '\(relation)', falling back to `.equal`.", file: file, line: line)
                    return anchor.constraint(
                        equalTo: space,
                        multiplier: multiplier,
                        constant: constant
                    )
                }
            })
    }
