Walker & Co. (Walker) signed a written contract to lease a large neon advertising sign to Herbert Harrison, who is in the dry-cleaning business, for $148.50 a month. The contract stated that Walker, as the lessor of the sign, would "at its expense maintain and service the sign [and would perform] cleaning and repainting of sign in original color scheme as often as deemed necessary by lessor to keep sign in first-class advertising condition and make all necessary repairs to sign and equipment installed by lessor." A few weeks the sign was installed, someone hit the sign with a tomato and "little spider cobwebs" appeared in the sign's corners. Harrison repeatedly asked Walker to fix the sign, but Walker did not do so. As a result, Harrison made no further payments and Walker sued Harrison for remainder of the lease payments pursuant to the contract's terms. Did Walker make a material breach of the contract?